Modelling plug flow

A central composite experimental design was used to evaluate the effects of amplitude (Xo), frequency (f) and net flow (Q) on the quality of plug flow achieved during continuous operation of a ‘standard’ OBR design by analysing residence time distribution profiles (RTD). The following were key findings:

 The tanks-in-series (TiS) model was shown to be a good representation of the flow conditions in an OBR.

 Mass balances demonstrated that >95% of tracer material introduced passed through the reactor indicating little stagnation.

 A second order polynomial model (R2=92.1%) was developed to predict the quality of plug flow from three variable factors (Xo, f and Q).

 Plug flow was maximised for Ψ=1.9 which is in the range previously identified by Stonestreet and van der Veeken (1999) (1.8<Ψ<2.0).

 Generation of plug flow is not entirely decoupled from the mixing intensity. Hence OBRs can still be “long” if plug flow is desired over a long residence time. However, they are still orders of magnitude shorter than conventional plug flow designs.

134 The final point above is important for development of commercial processes based on OBR technology because it demonstrates the need for consideration of OBR design in relation to process requirements. For example, not all OBRs can achieve plug flow for all processes, especially those with long residence times (>24 hours) such as many bioprocesses (e.g. enzymatic saccharification).

7.3 Enzymatic saccharification

Enzymatic saccharification of pure α-cellulose was conducted using OBR and conventional STR technologies over a range of mixing intensities, generating the following key findings:

 Reaction rates were mass transfer limited in both reactor designs at conditions of no or minimal mixing.

 The maximum conversion rate in the OBR was observed at a relatively low power density (2.36 W/m3) compared to the STR (37.2-250 W/m3).

 No evidence of shear inactivation was observed for STR runs due to a relatively low impeller speed compared to previous studies (Gunjikar et al., 2001, Ganesh et al., 2000).

 A comparison of the theoretical power densities required to achieve maximum conversion rates shows a reduction of 94-99% in the OBR (2.36 W/m3)

compared to the STR (37.2-250 W/m3).

 OBR technology could potentially increase profits by 2-14% compared to enzymatic saccharification processes based on STR technology.

The study demonstrated that OBRs are suitable for performing enzymatic saccharification reactions in a power-efficient manner compared to conventional STRs. However, a simple economic assessment with numerous assumptions suggested that the overall improvement would be 2-14% for a full scale process (2000 ton corn stover/day). This level of improvement is relatively low compared to the high risk associated with the design and manufacture of OBRs suitable for a full scale process. Commercial adoption of OBR technology is likely to require greater improvements that outweigh the risks associated with novel technologies.

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7.4 Microalgae culture

Chlamydomonas reinhardtii was grown in a modified OBR to test the technology for

use as a photobioreactor (PBR). The following were key findings:

 A flotation effect was observed without the need for addition of a frothing agent or surfactant.

 The OBR demonstrated a 95% increase in the average maximum growth rate compared to control cultures in T-flasks.

 Mixing intensity in the OBR had no effect on the maximum growth rate achieved, even with no mixing.

 Linear growth was observed in all cultures which indicates limitation.

The study demonstrated that OBR technology could be used for the liquid culture of microalgae. The tubular design is conducive to efficient harvest of sunlight and a closed system enables compliance with the necessary regulations and guidelines associated with GMO use and API manufacture. However, the mixing intensity was shown to have no effect on the growth rate achieved which suggests agitation caused by gas rising through the column is sufficient under these conditions. The main finding was a flotation effect which could enable development of a more economic process for the dual culture and harvest of microalgae cells.

7.5 Anaerobic digestion

This study compared the performance of two digester designs based on OBR and STR technologies for anaerobic digestion (AD) of dairy slurry and co-digestion with glycerol. The following were key findings:

 Feed with a particulate content was not suitable for this OBR design.

 Biogas production was enhanced by 43% in the OBR with continuous agitation compared to the STR with intermittent agitation.

 Destabilisation occurred in the STR with the addition of 1.4% glycerol to the feed which required a reduction in the feed rate and continuous agitation to prevent complete process collapse.

 The OBR was able to cope with a shock change in feed composition due a ‘buffer zone’ created by the tubular design.

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 Co-digestion with glycerol enhanced methane production by ~270% in both digesters compared to AD of dairy slurry.

 Maximum specific methane yields (SMYs) of 0.51 and 0.40 m3/kg VSadded were achieved at theoretical P/Vs of ~190 and 20 W/m3 in the OBR and STR,

respectively.

 At P/V=150 W/m3 the STR showed a significant reduction in the SMY, which indicates process destabilisation at moderate agitation intensities.

 The optimum organic loading rate (OLR) in both digesters was shown to be ~4.3 kg COD/m3 day which generated 0.51 and 0.40 m3/kg VSadded for the OBR and STR, respectively, compared to 6.44 kg COD/m3 day and ~0.59 m3/kg VSadded in the literature (Castrillón et al., 2013).

 89% less power consumption was required for temperature control in the STR compared to the OBR due to a reduction of 82% in the SA:V ratio which reduces heat loss.

These results demonstrate that OBR technology is capable of being used for AD and can equal or exceed the performance in terms of SMY of digesters based on STR technology. However, there are design issues with digesters based on OBR technology which include an increased SA:V ratio for heat loss; and potential blockages in ‘u-bends’ when using feed with a moderate particulate content.