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ReviewCite this article: Abbott MSR, Harvey AP,

Valente Perez G, Theodorou MK. 2013

Biological processing in oscillatory

baffled reactors: operation, advantages and

potential. Interface Focus 3: 20120036.

http://dx.doi.org/10.1098/rsfs.2012.0036

One contribution of 9 to a Theme Issue

‘Biofuels, science and society’.

Subject Areas:biotechnology, chemical engineering

Keywords:oscillatory baffled reactor, continuous

bioprocess, sustainable bioprocess,

predictable scale-up

Author for correspondence:M. S. R. Abbott

e-mail: matthew.abbott@ncl.ac.uk

& 2012 The Author(s) Published by the Royal Society. All rights reserved

Biological processing in oscillatorybaffled reactors: operation, advantagesand potential

M. S. R. Abbott1,3, A. P. Harvey2, G. Valente Perez3 and M. K. Theodorou3,4

1Bioprocessing Biopharmaceutical Technology Centre and, 2Chemical Engineering and Advanced Materials,Newcastle University, Newcastle upon Tyne, UK3The Centre for Process Innovation, Redcar, UK4Department of Biological and Biomedical Sciences, Durham, UK

The development of efficient and commercially viable bioprocesses is essen-

tial for reducing the need for fossil-derived products. Increasingly,

pharmaceuticals, fuel, health products and precursor compounds for plastics

are being synthesized using bioprocessing routes as opposed to more tra-

ditional chemical technologies. Production vessels or reactors are required

for synthesis of crude product before downstream processing for extraction

and purification. Reactors are operated either in discrete batches or, prefer-

ably, continuously in order to reduce waste, cost and energy. This review

describes the oscillatory baffled reactor (OBR), which, generally, has a

niche application in performing ‘long’ processes in plug flow conditions,

and so should be suitable for various bioprocesses. We report findings to

suggest that OBRs could increase reaction rates for specific bioprocesses

owing to low shear, good global mixing and enhanced mass transfer

compared with conventional reactors. By maintaining geometrical and

dynamic conditions, the technology has been proved to be easily scaled

up and operated continuously, allowing laboratory-scale results to be

easily transferred to industrial-sized processes. This is the first comprehen-

sive review of bioprocessing using OBRs. The barriers facing industrial

adoption of the technology are discussed alongside some suggested strat-

egies to overcome these barriers. OBR technology could prove to be a

major aid in the development of commercially viable and sustainable

bioprocesses, essential for moving towards a greener future.

1. IntroductionBioprocessing uses complete living cells or any of their components for the pro-

duction of useful products ranging from high value pharmaceuticals [1] to low

value fuels [2]. A growing interest in renewable technologies to replace

traditional fossil-derived chemicals with, for example, biomass [3] has stimu-

lated increased research and development targeting a range of bioprocesses.

The aim is to develop bioprocesses based on renewable and organic feedstocks,

with the constraint of maintaining or decreasing current production costs com-

pared with traditional technologies. This is challenging, given the decades of

optimization and intensification of chemical processes. In addition, biopharma-

ceuticals (e.g. trastuzumab) [4], nutraceuticals (e.g. astaxanthin) [5], CO2

capture [6] and protein refolding [7] use bioprocessing routes.

The entire production pathway from feedstock to product requires many

stages, including: pre-treatment, production, extraction and purification. Tra-

ditional batch stirred tank reactors (STRs) and continuously stirred tank

reactors (CSTRs) have existed for centuries and are still widely adopted

throughout the chemical and bioprocessing sectors for production owing to

their simplicity. In essence, STRs and CSTRs are nothing more than large

vessels mixed using a paddled shaft and, although suitable for many processes,

they lack specific characteristics essential for intensified and cost-effective bio-

processing. For example, achieving good global mixing complemented with

net flow oscillatory motion

reciprocating pump

L

DDo

d

(a)

(b)

(c)

material input

Figure 1. Standard oscillatory baffled reactor (OBR) design. (a) Tube containing equally spaced orifice plates (baffles) with a reciprocating pump required forgeneration of oscillatory motion. (b) Geometrical parameters important for OBR design. (c) Continuous operation requires a second pump to create net flow throughthe column.

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low shear is difficult in STRs: a combination essential for

specific bioprocesses including the culture of microalgae

that require mixing to provide illumination and CO2 but

suffer from cell fragility [8].

Continuous technologies for bioprocessing and biophar-

maceutical sectors have become more prevalent owing to

their ability to reduce footprint, waste, cost and energy com-

pared with batch technologies by, for example, removing

down-time inherent in batch processing [9]. Once at steady

state, a continuous process produces product with little

variation in output, providing variable factors such as temp-

erature, pH and feed constituents are kept constant.

Oscillatory baffled reactors (OBRs) allow the development

of continuous processes under plug flow conditions whereby

biological components move continuously through the reac-

tor with laminar flow. Plug flow in OBRs can, therefore, be

viewed as batch culture with the time dimension replaced

by reactor length. This enables bioprocesses containing cell

cultures to be extracted at the outlet, with cells at any desired

metabolic state to maximize product concentration. This

includes cultures with zero or negative growth rates, such

as those in the decline phase of batch culture: unachievable

using traditional continuous chemostats in CSTRs that rely

on net growth rates for dynamic stability [10].

2. Theory governing oscillatory baffled reactordesign and operation

2.1. Design2.1.1. The ‘standard’ designA ‘standard’ OBR consists of a tube, generally 10–150 mm

internal diameter (D), containing equally spaced orifice

plates (figure 1). Typically, a reciprocating pump or piston

located at one end oscillates back and forth, generating oscil-

latory flow. For continuous operation, a second pump is

required to create net flow through the column. Uniform

mixing at exceptionally low shear is provided by vortices

that form as fluid is forced through each orifice plate. OBRs

can act as either batch or continuous systems, depending

on whether a net flow of new material is being introduced

to the reactor and product removed at an equal rate.

2.1.2. Other designsAlthough figure 1 shows the most common design, other

OBR designs exist for scaling down (mesoscale) and up

(e.g. ‘multi-orifice’, see §4.2).

Mesoscale OBRs have a niche application for the rapid

screening and characterization of reactions (e.g. biodiesel for-

mation) [11]. These meso-reactors have a small diameter

(approx. 5 mm) and subsequently volumes of a few millilitres

[12]. Several baffle designs have been evaluated at the meso-

scale: helical [13,14], smooth periodic constricted tube [15],

central and integral [16].

2.2. Mixing through vorticesUnlike STRs and conventional tubular reactors that rely on

stirring mechanisms and/or turbulent flow conditions for

mixing [17,18], the OBR uses oscillations to produce vortices

(figure 2). These form periodically along the entire length of

the reactor, effectively causing each inter-baffle zone to act as

a CSTR; the entire reactor therefore consists of a finite number

of CSTRs connected in series. The key difference between a

conventional tubular reactor and an OBR is that mixing inten-

sity in the latter can be controlled, not by altering the flow

rate, but instead by changing the oscillating conditions,

impacting the size and frequency of vortex formation.

Solutions of the Navier–Stokes equations have been used

to calculate the flow patterns generated inside periodically

constricted tubes [19] and predict a two-phase cycle for oscil-

latory flow: during acceleration, vortices form behind

constrictions in furrows, growing until flow reversal when

they are forced into the mainstream flow and fade. These pre-

dictions were observed experimentally [20] and are relevant

furrow

(a)

(b)

mainstream flow

inter-baffle zone

Figure 2. Vortex formation in an OBR created by oscillatory flow. Vorticesform in the furrows during acceleration and are forced into the mainstreamflow during flow reversal. (a) Back stroke. (b) Forward stroke.

Table 1. Geometrical parameters important for oscillatory baffled reactordesign.

parameter symbol optimal value reference

baffle thickness d 2 – 3 mm [21]

baffle spacing L 1.5 D [22]

baffle open area a 20 – 22% [21]

orifice diameter Do 0.45 – 0.50 D [21]

tube diameter D usually 10 – 150 mm n.a.

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to OBRs because orifice plates produce constricted regions,

resulting in similar flow conditions when oscillated.

2.3. Geometrical parametersThe geometrical parameters, or physical dimensions,

important for OBR design are summarized in table 1 and

figure 1b. In total, there are five parameters that need to be

considered with the baffle spacing (L) and baffle open area

(a), defined as (Do/D)2, being the most important.

The shape and length of vortex formation are defined

by L. Generating uniform and effective mixing, vortices

require adequate room to fully expand and spread through-

out the inter-baffle zone. Suboptimal distances result in

vortices colliding with neighbouring baffles before full expan-

sion, leading to undesired axial dispersion when operating

continuously. Superoptimal distances lead to vortices that

do not propagate through the full volume of the inter-baffle

region, producing stagnant regions. The effects on mixing

of altering the baffle spacing using distances of 1, 1.5 and 2

times D have been evaluated [22]. A spacing of 1.5D gave

the most effective mixing over the greatest range of oscillation

amplitudes (Xo), and so has been standardized and used for

most subsequent work with OBRs.

The width of vortices formed within inter-baffle zones is

defined by a, with larger values giving rise to narrow vor-

tices and, consequently, poor mixing. By reducing the

orifice diameter (Do) fluid is constricted to a greater extent

as it passes through each baffle resulting in wide vortex for-

mation, generating effective mixing conditions. The effects

on mixing for 11 � a � 51 per cent have been evaluated

using the ‘mixing time’ defined as ‘the time measured from

the instant of tracer addition until the column contents has

reached a specified degree of uniformity’ [21]. Using 4 per

cent sodium hydroxide as tracer, 20 � a � 22 per cent was

found to minimize the mixing time. Baffle thickness (d) was

also evaluated in a 50 mm diameter OBR using thicknesses

of 1–48 mm [21]. It was found that thicker baffles resulted

in vortex deformation owing to an increased ‘cling time’

with the optimum thickness being identified as 2–3 mm.

2.4. Operational parameters2.4.1. Batch operationTable 2 summarizes the dimensionless groups and dynamic

parameters used in oscillatory flow.

The Reynolds number of oscillation (Reo) gives an indi-

cation of mixing intensity. Flow separation occurs when the

boundary layer becomes detached from a surface and forms

vortices—in this case, the OBR contents detaching from the

wall. For flow separation to occur in OBRs, Reo must

exceed 50–100 [23], as opposed to a net flow Reynolds

number (Ren) in conventional tubular reactors of 2100 [24].

An increase in Reo can be achieved in situ by increasing the

amplitude (Xo) or frequency ( f ) of oscillation, producing a

wide range of mixing intensities from ‘soft’ (50 � Reo �500), where vortex formation occurs, to the most intense

(Reo . 5000) corresponding to mixed flow with the OBR

acting as a STR [24].

The Strouhal number (St) is inversely proportional to Xo

and measures vortex propagation [21,25]. Large St values

are produced at small amplitudes, giving poor vortex for-

mation and vice versa. From studies available in the

literature, the tested range for St is 0.01–9 [22,26]; however,

the most common range used is 0.15–4 [27–29].

2.4.2. Continuous operationWhen operating continuously, Reo should dominate, giving

almost full reversal in flow, thereby creating vortices and

generating effective mixing. The value of the velocity ratio

(c) is a measure of the degree of plug flow achieved [30]

and can be evaluated using the tanks-in-series (TIS) model

for plug flow [31].

The TIS model assumes that flow conditions can be rep-

resented by a variable number of ideal CSTRs in series (Nt).

As Nt increases, the predicted residence time distribution

(RTD) curve during a pulse test approaches one that

would be observed during perfect plug flow. RTD is the

probability distribution that describes how long material

could spend in a reactor. Mixed flow is characterized by

an RTD with a peak followed by a steady decline, whereas

plug flow is symmetrical about the mean residence time.

Mixed and plug flow describe two extreme RTDs achievable

but, in reality, the true flow condition lies somewhere in

between. If Nt is infinite, then all molecules leave the reactor

with identical residence times, a characteristic of perfect

plug flow. Achieving approximations to plug flow is ben-

eficial for processes that require precise residence times or

removal of back mixing.

Table 2. Dimensionless groups and dynamic parameters required for oscillatory baffled reactor operation.

parameter symbol equation description

centre to peak amplitude of oscillation (m) Xo n.a. half the fluid oscillation distance

frequency of oscillation (Hz) f n.a. number of oscillations per second

volumetric flow rate (e.g. ml min21) Q n.a. volume of material entering the OBR over a given time period

Reynolds number of oscillation Reo (r.2pf.Xo.D)/m a measure of mixing intensity

Strouhal number St D/4p.Xo a measure of effective vortex propagation

net flow Reynolds number Ren (r.D.u)/m a measure of the net flow

velocity ratio c Reo/Ren the ratio of Reo to Ren

Table 3. Advantages provided by OBRs over conventional CSTRs and tubular reactors.

advantage description benefit reference

uniform mixing the vortex cycle creates equal radial transfer

across the tube producing uniform mixing

patterns throughout the OBR, reducing heat

and concentration gradients

the removal of gradients can reduce the overall

reaction time; e.g. up to 18-fold reduction for

speciality chemicals

[32]

low and uniform shear low, uniform shear in the OBR compared to

STRs makes the reactor more suitable for

shear-sensitive organisms and large

molecules

up to a 10-fold reduction in shear rates

compared to STRs. Generates increased particle

flotation

[33,34]

increased mass transfer uniform bubble size and increased gas hold up

produce enhanced mass transfer rates

sixfold and 75% increases for oxygen transfer

(kLa) into water and yeast culture, respectively

[28,35]

compact reactor design the ability to generate long residence times,

under plug flow, with reduced reactor

lengths allows compact designs

up to a 600-fold decrease in reactor length

compared with conventional tubular reactors

[36]

linear scale-up maintaining St, Reo and Ren allows mixing

intensity and flow conditions to be predicted

in large volume OBRs using data from

laboratory-scale experiments

laboratory-scale experiments can be scaled up by

increasing OBR diameter or length while

maintaining predictability

[29,37,38]

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Experimental RTD profiles have been produced by inject-

ing 3 M aqueous potassium chloride into a 1.3 l OBR with a

net flow of deionized water under varying Reo and Ren

[30]. By comparing with theoretical RTD profiles, Nt was

evaluated and plotted against c. The range of 2 � c � 4

maximized Nt and generated optimal plug flow conditions;

however, useable degrees of plug flow can still be achieved

using values of c between 2 and 10. Using values of c

below 2 results in loss of the major design advantage of

achieving plug flow in reduced length reactors for long resi-

dence time processes, whereas values of c above 10 lead to

loss of plug flow.

3. Bioprocessing advantages3.1. OverviewThe OBR offers numerous advantages over conventional

reactors that not only allow development of continuous

bioprocesses, but can also enhance reaction rates and

productivity during batch operation. Some of these advan-

tages are unique to culturing living organisms that require

key nutrients for maximum growth. For example, living

cells often require oxygen (e.g. yeast) or carbon dioxide

(e.g. microalgae) to grow and can be shear-sensitive because

of their relative fragility and large size. Table 3 summarizes

these major advantages.

3.2. Reduced shear rateShear is an important factor for bioprocesses involving cells

or large molecules, such as enzymes, which can be inhibited

by high shear rates. The biological definition is given as ‘the

rate of change of velocity at which one layer of fluid flows

over an adjacent parallel layer, often expressed in seconds21’

[39]. Particle image velocimetry has been used to record the

shear rate distribution in a 50 mm diameter OBR, and a

close correlation between Reo and the mean shear rate

(gOBR) was observed [33].

The average shear rate generated in STRs is proportional

to the impeller speed (N ), although this relationship differs

0

10

20

30

40

50

60

70

80

5 10 15 20 25 30 35 40 45 50

STR = 300 rpmOBR = 4000 Reo

shea

r ra

te (

s–1)

power density (W m–3)

Figure 3. Theoretical average shear rates for a 50 mm OBR (solid line) and 2 lSTR (dashed line) during unaerated operation at increasing power densities [33].

100 200 300 400 500 6000

5101520253035404550

STR = 400 rpmOBR = 5000 Reo

power density (W m–3)

k La

(h–1

)

Figure 4. kLa against increasing power density for a 50 mm OBR (solid line)and 2 l STR (dashed line) at a constant aeration rate [28].

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according to various authors [40–42]. A comparison is shown

in figure 3 using the lowest estimation of shear for a 2 l STR

[41], previously used for comparison against a 50 mm OBR

(251 � Reo � 4021) [33]. The average shear rate has been

plotted against power density (P/V): a measure of energy

being applied to a system, expressed in W m23.

Figure 3 demonstrates that the average shear rate is much

higher in STRs: at 40 W m23, the OBR shows a fivefold

reduction. Periodic shedding of vortices and vortex–vortex

interactions provide uniform shear, controlled by the oscil-

lation conditions. The flow patterns in OBRs are such that

radial and axial flows are of similar magnitude. This leads

to a more uniform shear field. In particular, the high shear

points around impellers in typical STRs are absent. High

shear near the impeller in STRs can damage micro-organisms

even if the experience is brief. Mammalian and insect cells

have been reported to be shear-sensitive [43,44], as well as

cellulase enzymes, which are important for saccharification

reactions [45–48], and microalgae [8]. The OBR offers a

viable alternative for bioprocesses inhibited by high shear

rates produced in STRs, where a degree of mixing is required

to achieve sufficient mass transfer.

Shear uniformity throughout a reactor benefits biopro-

cesses where larger particles, millimetres in size, are being

used. For example, during flotation, particles must be

suspended and sedimentation avoided. Flotation is a separ-

ation technique whereby material is ground into fine

particles before being made hydrophobic by surfactant

addition. Once suspended in an agitated tank sparged with

air, the hydrophobic particles attach to rising air bubbles,

forming a surface froth to be concentrated and purified.

Fine quartz particles, 3–104 mm in size, rendered hydro-

phobic using dodecylamine were used to test the suitability

of an OBR for use as a flotation device [34]. A 60 per cent

improvement in flotation for finer particles, less than

30 mm, and 30–40% for coarse particles up to 104 mm at

much lower power densities compared with conventional

flotation devices were reported. Compared with Rushton

impeller agitated flotation devices that can have up to

30–40 times the energy dissipation close to the impeller

compared with the bulk of the vessel [49], OBRs have been

shown to have a more even distribution of shear [33], explain-

ing the improvements in flotation. This strongly suggests that

OBRs are suitable for performing bioprocesses containing

biomass particles that need to be retained in suspension

without sedimentation.

3.3. Enhanced mass transferMany bioprocesses use aerobic organisms to produce useful

products such as polyhydroxyalkanoates (PHA), precursors

for bioplastics, synthesized by the aerobic bacterium

Pseudomonas putida [50]. During culture of these organisms,

air must be sparged into the reactor providing oxygen. In

some cases, the oxygen uptake rate becomes higher than

the maximum achievable oxygen transfer rate (OTR), result-

ing in oxygen limitation. One method to overcome this is to

sparge with pure oxygen, but this creates additional safety

issues and adds cost. However, OBRs offer an alternative sol-

ution to increasing OTRs with sixfold increases in kLa for

oxygen transfer into water reported compared with conven-

tional STRs [35].

Mass transfer of gases, in particular oxygen, into liquids is

usually quantified using kLa: ‘the volumetric mass transfer

coefficient that describes the efficiency with which oxygen

can be delivered to a bioreactor’ [51]. Rate of mass transfer

of gas is a function of the mass transfer coefficient of the

specific gas in question and the surface area available for

transfer into the bulk liquid medium (which itself is a func-

tion of both bubble size and hold-up time). The kLa values

for a 2 l STR and 50 mm diameter OBR during the fermenta-

tion of yeast cells, Saccharomyces cerevisiae, have been

determined [28]. A comparison of kLa against power density

using these data is shown in figure 4. At a power density of

100 W m23, intense mixing conditions are produced in both

reactor types. However, the kLa value produced in the OBR

is approximately 75 per cent higher than the STR: predomi-

nantly a result of enhanced gas hold-up time but also

reduced bubble diameter [52]. The unique fluid mechanics

in OBRs produce a longer path length for individual gas

bubbles, thereby increasing gas hold-up, by increasing each

bubble’s residence time. Vortex interaction with gas bubbles

causes break-up producing a larger surface area for gas trans-

fer. These characteristics allow OBRs to maintain sufficient

oxygen transfer with good mixing at low shear. Achieving

similar values for kLa in STRs would require reactor modifi-

cation [53], increased impeller speeds (producing increased

shear rates) or a switch to pure oxygen.

3.4. Compact design for plug flowTraditionally, plug flow has been achieved on an industrial

scale by either connecting a series of CSTRs together or using

tubular reactors under turbulent flow conditions [32]. On a

37.5 mm

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practical level, there is a limit to how many CSTRs can be con-

nected in series or how long a reactor can be. In OBRs, mixing

intensity is decoupled from the flow rate allowing long resi-

dence time processes in reduced length reactors. For example,

it has been calculated that for a flow rate of 2381 l h21 and

residence time of 4 h, the required reactor lengths for an OBR

and conventional tubular reactor are 1213 and 757 894 m,

respectively [36]. The OBR offers an alternative process intensi-

fication methodology that provides a high degree of plug flow

in a reactor with a much reduced length [30].

Table 4. Required operating conditions to maintain St and Reo at 1.0 and500, respectively.

tube diameter(mm)

requiredXo (mm)

requiredf (Hz)

25 1.99 1.6

50 3.98 0.4

150 11.94 0.04

12.5 mmorifice

100 mm 25 mm

Figure 5. Multi-orifice baffle design creating the effect of 16 ‘standarddesign’ 25 mm diameter OBRs operated in parallel for a 100 mm diameterreactor [29].

InterfaceFocus

3:20120036

4. Scale-up4.1. Direct diameter increasesA promising aspect of OBR technology is the ability to

scale-up processes by maintaining geometrical and dynamic

similarity, allowing mixing and flow conditions produced

at laboratory scale to be easily replicated for pilot and indus-

trial-scale processes. St, Reo and Ren are assumed to fully

define the fluid dynamic conditions for a particular OBR geo-

metry [37]. By keeping these parameters constant, an OBR

with a diameter of, for example, 24 mm should behave the

same as one with a diameter of 150 mm.

Axial dispersion coefficients (Dc) have been used to study

the effects of tube diameter on the mixing and flow con-

ditions of three OBRs with 24, 54 and 150 mm diameters

[37]. An imperfect pulse technique was adopted and the

pulse concentration measured at a minimum of two points

downstream. By comparing the two RTD profiles, Dc was cal-

culated using the dispersion model [31]. The model assumes

that a diffusion-like process is occurring that is superimposed

on to plug flow resulting in axial spreading of material. Dc

represents the extent of spreading with large values equating

to rapid spreading and small values to slow spreading, or

closer approximations to true plug flow. Methylene blue

was selected as the tracer owing to its high optical density

at low concentrations and measured using optical sensors

placed at known distances from the site of injection.

Three distinct flow regimes were indentified: for Reo , 80,

Dc tends towards 5 � 1024 m2 s21; for 80 , Reo , 800 axial

dispersion was minimized with values for Dc as low as

1024 m2 s21; and for Reo . 800, Dc increases approximately

linearly with Reo [37]. Optimal interaction between the net

flow and oscillatory mixing was identified as the reason for

the minimum: at 80 , Reo , 800, the vortices created were

optimal for radial redistribution of dye, thereby minimizing

axial dispersion [29]. It was established that axial dispersion

was not a function of D, indicating that fluid dynamic con-

ditions could be maintained during scale-up providing St,Reo and Ren were kept constant.

4.2. Multi-orifice designDuring scale-up of OBRs, a doubling of the reactor diameter

must be complemented with a doubling in Xo to maintain Stat a specified value. This results in Xo being fixed, leaving

only f as a variable operational parameter to control Reo.

Table 4 summarizes the values of Xo and f required to main-

tain St and Reo at 1.0 and 500, respectively, during scale-up

from 25 to 150 mm diameter.

At higher diameters, the frequency of oscillation must be

extremely low, which reduces the mixing intensity and the

opportunity for improved mass transfer [29]. To overcome

this problem, a method of scale-up involving a bundle of rela-

tively small diameter OBRs operated in parallel, thereby

removing the need for extremely low frequencies, has been

proposed [54]. However, this solution produces two other pro-

blems: how to maintain an equal distribution of flow to each

separate tube, and generating equal oscillating conditions.

A different approach has been adopted whereby baffles

containing multiple orifices are used, with D being replaced

by the effective tube diameter (De), calculated by dividing

the total baffle area by the number of orifices. It was pre-

dicted that a 150 mm diameter OBR with internal baffles

consisting of 37 orifices would behave in a similar way to

a 24 mm diameter OBR [37]. This was demonstrated by

recording similar axial dispersions at increasing Reo, while

maintaining Ren at 107, in both 24 mm and multi-orifice

150 mm OBRs [37]. The major advantage of this design is

that the same shear rates and intensity of mixing achieved

in a smaller diameter OBR can be maintained while greatly

increasing the throughput of the process (per unit length of

reactor). Multi-orifice designs are particularly attractive

owing to the ease of manufacture and ability to maintain

fluid mechanics and axial dispersion, allowing experiments

conducted at laboratory scale to be increased to industrial

volume with predictability of axial dispersion and mixing

intensity [29]. Figure 5 depicts a multi-orifice baffle design

(100 mm in diameter) that produces characteristics observed

in a conventional 25 mm OBR.

Table 5. OBR bioprocesses using cellular components.

bioprocess cellular component year conclusion process type reference

protein refolding hen egg white

lysozyme

2001 OBRs are suitable refolding devices and

comparable to STRs

non-cell culture [7]

protein renaturation hen egg white

lysozyme

2002 refolding yield in OBR comparable to STR.

Uniform shear important for successful

refolding

non-cell culture [72]

protein refolding chicken egg white

lysozyme

2006 prevention of aggregation enhances

refolding during initial stages (0 – 4 min)

non-cell culture [73]

saccharification cellulase from

Trichoderma reesei

2011 7% increase in glucose production after

48 h compared to shake flask

enzymatic [74]

Table 6. OBR bioprocesses using anaerobic cell cultures. Asterisks denote patent pending.

bioprocess organism year conclusion cell type reference

flocculation Alcaligenes eutrophus 2001 higher degree of flocculation compared

to STR at lower polymer dose. St

the dominant factor

bacterial [75]

Acetone, butanol

and ethanol

fermentation

Clostridium acetobutylicum 2009 115% increase in solvent production in

OBR from 0 – 0.78 Hz. 90% increase

in butanol compared with STR

bacterial [56,76,77]*

anaerobic process patent for culture of facultative/

obligate anaerobes in OBRs

2010 patent publication bacterial [78]*

ethanol production Saccharomyces cerevisiae 2011 9% increase in ethanol production after

48 h compared with shake flask

yeast [74]

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5. Bioprocessing in oscillatory baffled reactors5.1. OverviewTypical chemical processes can take anywhere from fractions

of a second to many hours in a conventional reactor. The pro-

cess advantages of OBRs have been investigated and

described for a range of applications including biofuel

production [55,56], flotation [34], butylation of phenylaceto-

nitrile [57], oil droplet breakage [58–60], photo-oxidation

[61,62], polymerization [63] and extensive work on crystalli-

zation [64–71]. However, this review aims to target

bioprocesses; so chemical reactions will not be discussed in

any further detail.

Tables 5–7 summarize the current bioprocesses con-

ducted in OBRs available in the literature, all being

performed in batch mode. They have been grouped into

those processes that use a cellular component such as an

enzyme (table 5) and those using living cells, under both

anaerobic (table 6) and aerobic (table 7) conditions.

5.2. Bioprocesses using cellular componentsThree of the four bioprocesses described in table 5 are related

to protein refolding: a key unit operation when producing

recombinant biopharmaceuticals from expression systems

such as Escherichia coli. Most protein refolding operations

are optimized with respect to the chemical environment

[72]; however, the mixing environment also impacts on

refolding yield [7,83]. The preferred protein refolding

method at industrial scale remains direct dilution in STRs,

mainly because of its simplicity and widespread use. How-

ever, as STRs are scaled from laboratory to industrial

volumes, the mixing efficiency declines [7], impacting nega-

tively on protein refolding yield.

The table demonstrates that protein refolding can be per-

formed in OBRs with yields comparable to STRs at laboratory

scale. The lack of improvement does not suggest, however,

that OBRs offer no additional benefit to this bioprocess. The

major advantage on offer is a scalable mixing environment

[37], allowing yields obtained in the laboratory to be predic-

tably replicated on an industrial scale: currently unachievable

using STRs. As a result, higher yields are possible at large

scale during protein refolding bioprocesses, reducing overall

production costs.

Lignocellulosic materials are ubiquitous in nature, with cel-

lulose (the support molecule in plants) being the most

abundant carbohydrate on Earth. It is possible to hydrolyse

cellulose (saccharification) using various chemical or biological

methods, thereby liberating the glucose monomers that consti-

tute its structure, which can be fermented into a variety of

useful chemicals including ethanol and lactic acid [84,85].

During the enzymatic conversion of cellulose into monosac-

charide, cellulase deactivation occurs. This deactivation of

cellulases is caused by a number of process-dependent factors:

Table 7. OBR bioprocesses using aerobic cell cultures.

bioprocess organism year conclusion cell type reference

animal cell

culture

unknown 1991 feasible to culture animal cells in OBRs mammalian [79]

PHB production Alcaligenes eutrophus

H16

1992 feasible to culture rapidly growing, O2

demanding micro-organisms in OBRs

bacterial [79]

yeast culture Saccharomyces

cerevisiae

1995 75% increase in OTR (kLa) compared

with STR

yeast [28]

pullulan

production

Aureobasidium pullulans

IMI 145194

2005 pullulan concentrations of 11.3 and

12.1 g l21 take 96 and 144 h in STRs

compared with 37 – 39 h in OBR

fungal [80]

g-decalactone

production

Yarrowia lipolytica W29 2006 50% time reduction for maximum

g-decalactone concentration compared

with other scaled-down reactors

yeast [81]

yeast growth rate Saccharomyces

cerevisiae

2006 biomass increases of 83 and 214% in OBR

when compared with STR and shaken

flask, respectively

yeast [82]

PHA production Pseudomonas putida 2009 56% increase in biomass after 25 h

compared with STR

bacterial [50]

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shear inactivation [45–48], sugar inhibition [86], ion strength

[87], temperature [88] and formation of inert enzyme substrate

complexes. A further factor involved in cellulose depolymeri-

zation is the changing nature of the substrate over time; the

easily hydrolysable amorphous regions are digested first leav-

ing the recalcitrant crystalline regions [89]. Saccharification has

been conducted in a 25 mm diameter OBR using Xo and fvalues of 3 mm and 3 Hz, respectively [74]. The substrate

used was pure microcrystalline cellulose at a loading of 2.5%

w/v. An enzyme loading of 40 filter paper units per gram

of cellulose and 10 per cent b-glucosidase was used. The

results of the study showed an increase in the glucose yield

of 7 per cent after 48 h compared with a shake flask run

under the same conditions. The increase in glucose production

in the OBR was attributed to a ‘better mixed hydrolysis

environment’ [74]. In other words, the uniform and effective

mixing under low shear rates allowed the cellulase enzymes

access to substrate, increasing glucose production. A further

possible benefit is the reduced shear rates present in OBRs.

One of the factors contributing to cellulase deactivation is

shear [45–48] which, if reduced, will result in cellulases retain-

ing their activity for longer generating increased glucose.

However, as the comparison was made with a shake flask,

this requires further investigation.

5.3. Anaerobic bioprocessesTable 6 lists the anaerobic bioprocesses previously conducted

in an OBR.

Flocculation is a process by which small particles aggre-

gate, with the aid of a polymer, to form flocs large enough

to settle or be filtered: commonly used industrially, for

example, in water and wastewater treatment [75]. Tradition-

ally, STRs known as stirred tank flocculators are used for

this process to provide agitation essential for generating par-

ticle collisions. An OBR has been used as a flocculation device

for the bacterium Alcaligenes eutrophus and compared with an

STR, assessing the percentage flocculation at various operat-

ing conditions [75]. Although 100 per cent flocculation was

not reached in the OBR, a comparison with another study

[90] demonstrated that, for similar starting bacterial concen-

trations, fuller flocculation was achieved in the OBR at

much lower shear rates. Previous authors have commented

on the non-homogeneous nature of STRs that results in floc

break-up near the impeller zone where shear rates can be

two orders of magnitude higher than the average [91]. The

more even shear distribution in OBRs [33] allows flocculation

to occur at much lower average shear providing an attractive,

alternative flocculation device to STRs [75].

Acetone, butanol and ethanol (ABE) fermentation and

bioethanol production have been extensively covered in a

previous review [56] and so will not be discussed in any

further detail. No data currently exist for anaerobic digestion

using OBRs as the reference is to a patent pending [78]. The

patent describes a system for using OBRs as a generic plat-

form for culture of facultative and obligate anaerobes.

Volatile fatty acids and methane were produced using

microbial consortia from the rumen. The culture of gut

fungi was also highlighted in the patent [78].

5.4. Aerobic bioprocessesTable 7 lists the aerobic bioprocesses previously conducted in

an OBR.

Alcaligenes eutrophus H16 has commercial interest for pro-

duction of the biodegradable plastic poly-b-hydroxybutyrate

(PHB). This bacterium has been cultured in an OBR [79] with

a maximum specific growth rate (mmax) of 0.39 h21 compared

with 0.36 h21 and 0.35 h21 when using Erlenmeyer flasks at

10 per cent and 40 per cent working volumes, respectively.

OBRs are, therefore, suitable for culturing rapidly grow-

ing, oxygen demanding micro-organisms. The same paper

Table 8. Key barriers facing adoption of OBR technology.

design

lengths hundreds of metres required for fully continuous operation

with residence times of hours to days

pressure drop due to frictional losses will dampen oscillations

gas bubbles during aerobic operation may dampen oscillations and

disrupt plug flow

multiple sparging and feeding points required down OBR length

fouling of baffles and internal surfaces due to micro-organism

adhesion

large volumes required

conservative nature of bioprocessing industry

increased complexity of OBR technology compared with other

bioreactor designs

no industrial-scale OBRs dedicated to bioprocessing exist

lack of data for industrial-scale bioprocessing using OBRs

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alluded to the use of OBRs for culturing animal cells: an inter-

esting proposal as they are notoriously shear-sensitive [92]

with cell death being proportional to energy input; so any

scaled-up reactor should minimize energy input [43]. Biopro-

cesses containing animal cells could greatly benefit from the

low shear, high mass transfer environment produced in

OBRs to minimize shear while maintaining sufficient OTRs.

The fruity, peach-like aroma compound g-decalactone

has applications in the food industry as a flavouring agent

and can be biologically produced from the yeast Yarrowialipolytica. Micro-reactors are important tools for rapidly

screening and optimizing bioprocesses [81]. A mesoscale

OBR (D ¼ 4.4 mm) has been used to culture Y. lipolytica for

the production of g-decalactone [81]. The processing time

required to reach maximum g-decalactone concentration in

the OBR was 50 per cent lower compared with traditional

scaled-down platforms: STRs and shake flasks. Enhanced

mass transfer rates were reported as the reason for the

observed reduction in processing time. The same mesoscale

OBR was used to culture S. cerevisiae, with an increase in bio-

mass of 83 per cent, using 93.6 per cent less air, compared

with a scaled-down STR [82]. The use of mesoscale OBRs

for rapid screening and optimization has two major advan-

tages: precise control over mixing and mass transfer; and

optimizations achieved at this scale can be predictably

scaled up to industry volumes.

Aureobasidium pullulans IMI 145194 was cultured in a

100 mm diameter OBR, with a working volume of 2.5 l,

using fixed Xo and f values of 20 mm and 2 Hz, respectively.

The aeration rate was optimized and then kept constant at

1.0 vvm (volume of air per unit volume of medium per

minute). The results showed that production of the versatile

biopolymer pullulan occurred during the exponential and

stationary phases, reaching a concentration of approximately

11.7 g l21 after 38 h. These values were compared with STR

data available in the literature [93], indicating that to reach

a comparable pullulan concentration in 2 and 10 l STRs, the

fermentation time must be increased to 96 and 144 h, respect-

ively. This equates to a reduction in the required processing

times of 60 per cent and 74 per cent when using OBRs as

opposed to STRs and highlights the problems encountered

when scaling up STRs. The move from 2 to 10 l using STRs

has resulted in a 50 per cent increase in the required

processing time to reach similar product concentrations.

No reasons were given for possible causes of increased

pullulan production but it seems likely that both effective

oxygen transfer and low shear rates contribute. It is not

entirely clear which of these factors is more important how-

ever; previous studies have shown that pullulan production

increased at higher impeller speeds and, therefore, high

oxygen transfer but also higher shear [94,95]. These are

contradicted by another study suggesting that low impel-

ler speeds and low shear are optimal [96]. It has been

demonstrated that an assisted airlift reactor designed for

maximum oxygen transfer and mixing produced a significant

increase in pullulan production from A. pullulans [97]. These

studies suggest that both high oxygen transfer and low shear

are most effective for pullulan production. Both of these con-

ditions are hard to achieve simultaneously in conventional

STRs because any increase in the impeller speed for increased

oxygen transfer is accompanied by an increase in shear.

Figures 3 and 4 show, however, that an OBR can achieve

both of these conditions, which could explain the increased

pullulan production observed, highlighting the potential of

OBRs for this type of bioprocess.

6. Industrial implementationThe ultimate niche application for the technology would be in

continuous bioprocessing under plug flow conditions,

removing down-time inherent in batch processing and redu-

cing plant footprint as a result of compact reactor design.

However, the authors are aware that in reaching this goal,

several key barriers must be overcome in both the complex

design of a large OBR and the conservative attitude prevalent

in the bioprocessing industry.

6.1. BarriersTable 8 highlights the key barriers facing adoption of OBR

technology by the bioprocessing industry.

Bioprocesses typically require residence times in excess of

24 h. Standard tubular plug flow reactors would need to be

thousands of metres in length compared with the OBR’s

hundreds of metres. Nevertheless, over these distances, trans-

mission of oscillations, gas sparging and maintenance of a

compact design remain problematic. There are also issues

with fouling and the requirement to process large volumes,

both relevant to any bioprocess using micro-organisms.

There are currently no industrial bioprocessing facilities

using OBR technology; so it has not yet been proved at this

scale. The issue is that for industry to invest time and money

developing an OBR capable of performing a continuous biopro-

cess with a residence time of hours or days, there must be solid

evidence supporting the benefits of doing so. This evidence is

currently based on smaller pilot and laboratory-scale exper-

iments in batch because even at reduced diameters, the length

must still be hundreds of metres to support fully continuous

operation: unlikely to occur in a university or small research com-

pany. Experimental evidence is therefore based on batch

experiments (focusing on low shear and enhanced mass transfer)

Table 9. Summary of findings.

the OBR tubular reactor, containing orifice plates

capable of batch or continuous operation

reciprocating piston or pump creates

oscillatory flow forming vortices for

effective mixing

process advantages low and even distribution of shear

compared to STRs

increased mass transfer, specifically gases

including O2

good global mixing

enhanced reaction rates for specific

bioprocesses

bioprocesses inhibited by mass transfer,

high shear and heterogeneity are

likely to benefit more from OBRs

continuous operation plug flow achievable over a range of

laminar flow rates

compact plug flow design compared

with conventional tubular reactors

scale-up predictable scale-up (for batch and

continuous) from laboratory to

industrial scale by maintaining

geometrical and dynamic similarity

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with advantages from continuous operation and predictable

scale-up being theoretical.

6.2. Recommended strategies6.2.1. Design solutionsObviously, a straight tube hundreds of metres in length

would be impracticable. A compact reactor design can be

maintained using a serpentine shape consisting of numer-

ous short sections connected using baffled ‘U-bends’: a

successful design used at laboratory scale. The benefit of

this design is that it provides the opportunity to add

numerous oscillators ensuring transmission of oscillations

down the reactor, which would otherwise diminish over

the lengths required. Vertical orientation enables gas spar-

ging at the base (and removal at the top) of each column

for aerobic bioprocesses. A major issue that needs addres-

sing is how the flow conditions would be affected in

those columns where the internal fluid is flowing against

rising gas bubbles. Numerous oscillators and sparging

points add complexity and possible contamination sites to

the design and must be weighed against the benefits

OBR technology would bring to the bioprocess.

Industrial-scale bioprocesses require large volumes to

be processed as product titres can be as low as 2 g l21 [1].

Current scale-up using STRs is complex owing to different

mixing patterns occurring at scale. Conventional STRs are

prone to impeller flooding, a phenomenon characterized by

gas rapidly flowing axially upwards past the impeller with

no radial discharge, and the formation of stagnant zones

[98,99]. There are also other acknowledged limitations when

using STRs including gas channelling, resulting in reduced

gas dissolution and poor bulk mixing [17,100]. This requires

implementation of different scale-up strategies depending on

the specific bioprocess [101,102]. By lengthening an OBR or

increasing the diameter (see §§4.1 and 4.2), volumes can be

increased while maintaining the mixing environment. This

allows large volumes to be processed either in batch or con-

tinuously under plug flow: the ultimate niche application of

the technology not achievable in CSTRs.

It is possible that micro-organism adhesion to baffles and

internal surfaces will present fouling issues, as occurs in other

bioprocesses. Careful selection of construction material could

mitigate this problem, but it is likely that periodic cleaning

will need to take place depending on the extent and rapidity

of fouling.

6.2.2. Selecting a model bioprocessThe range of bioprocesses highlighted in tables 5–7 demon-

strate the variability in improvement witnessed from OBRs. It

is imperative that each bioprocess is evaluated on a case-by-

case basis to ensure the technology is being used to its full

potential. The key operating advantages available from OBRs

are low shear, enhanced mass transfer, scalability and continu-

ous operation under plug flow. Selection of a bioprocess that

benefits from at least one (and preferably more) of these is

vital to ensuring the correct application of OBR technology.

However, we have already stated that proving the benefits

of continuous operation and predictable scale-up is difficult

without development of long OBRs. This has resulted in all

experiments being batch comparisons benefitting purely

from low shear and enhanced mass transfer. Therefore, a

shear-sensitive organism (or component) requiring aerobic

conditions will show the greatest improvements during batch

operation (e.g. pullulan production [80]). Several anaerobic

bioprocesses have been conducted in OBRs and, with the

exception of ABE production, have shown marginal improve-

ments with the main justification for using the technology

being scalability. In comparison, four aerobic bioprocesses wit-

nessed a greater than 50 per cent improvement providing

greater incentive for industry to adopt OBRs [50,80–82]. It is

difficult to predict those bioprocesses that will benefit from

continuous operation and predictable scale-up at this stage.

6.2.3. Open access facilitiesOpen access facilities provide equipment capable of testing

various bioprocesses at scale. Large companies could use

these facilities to gather data assessing the benefits of OBR

technology for their specific bioprocess. Such facilities as

the Centre for Process Innovation on Teesside in the UK

already house a number of OBRs available for industry-

focused research. The next stage is to develop an OBR capable

of fully continuous operation with at least a 24 h residence

time to generate solid data on the benefits of predictable

scale-up and continuous operation for specific bioprocesses.

7. SummaryOBR technology provides a novel production vessel for

bioprocessing over a wide range of cellular components and

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micro-organisms. The reactor has several advantages over con-

ventional STRs: good mixing complemented with low shear;

increased mass transfer rates; linear and predictable scale up;

and continuous operation under plug flow conditions. As

mixing intensity is controlled by oscillating conditions, long

residence time processes required for biological reactions are

possible in relatively short OBRs compared with conventional

tubular reactors that rely on high flow rates to achieve mixing.

The advantages OBR technology could bring to biopro-

cessing are evident; however, issues remain regarding the

design and uptake of industrial-scale OBRs. Reactors hun-

dreds of metres in length will be required to realize the

ultimate goal of using OBRs for continuous bioprocessing

under plug flow conditions. Over these distances, it is

likely that multiple oscillators and sparging points will be

required and it is unclear as to how rising gas bubbles will

interact with internal fluid, possibly disrupting plug flow.

Open access facilities could prove essential in providing

industry with OBRs capable of testing bioprocesses in a con-

tinuous fashion on large scale: currently not possible using

small laboratory-scale OBRs.

Nevertheless, work to date has demonstrated the ability

OBRs have to enhance product production during bioproces-

sing, moving closer towards developing viable replacement

technologies based on sustainable, biological systems. More

research is required to identify those bioprocesses that could

be greatly intensified through OBR technology and funding

provided to develop industrial-scale systems, operated

continuously. Table 9 summarizes the findings of this review.

The majority of funding enabling this research has been provided byan EPSRC grant (EP/G037620/1) in biopharmaceutical and bio-process development granted to M.S.R.A. The authors thankNewcastle University and the Centre for Process Innovation for pro-viding technical expertise and funding, which enabled this research.The authors also thank the three anonymous reviewers for theirvaluable and detailed feedback on the manuscript.

036

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