Design considerations

A major advantage of OBR technology is the ability to scale up in a linear and more predictable manner (Smith and Mackley, 2006, Smith, 1999) compared to STR technologies where numerous scale up methodologies exist (Junker, 2004). Smith (1999) was able to demonstrate that multi-orifice baffles could be used to maintain the conditions achieved in small diameter OBRs (10-100 mm diameter) in those with much larger diameters (>150 mm) by simulating the effect of numerous OBRs operated in parallel, another scale up methodology (Ni, 1994). By adopting the multi- orifice baffle scale up approach, the SA:V ratio of OBRs could be significantly reduced thereby decreasing the power consumption required for temperature control. Furthermore, it is likely that OBRs with increased diameters could process feed with a relatively high particulate content due to the removal of constricted regions formed by ‘u-bends’. Figure 6.15 shows the design of a 200 mm diameter OBR which could be operated as 16 individual 50 mm diameter OBRs in parallel.

OBRs scaled with multi-orifice baffles could be designed to have diameters approaching those of comparable STRs which would result in equal SA:V ratios and remove the power consumption issue identified for temperature control. This would enable replication of the agitation environment at larger scale, however; the OBR would lose its ‘buffer zone’ and become sensitive to ‘shock’ changes in feed composition. Plug flow would also not be possible, which prevents separation of the process stages and subsequent optimisation to maximise methane production. A balance therefore exists between power consumption and simple design on one hand; and process separation through plug flow and a ‘buffer zone’ on the other for digesters based on OBR technology. The OBR used in this study has achieved and sometimes exceeded methane yields obtained from a more conventional STR and was able to cope with shock changes in feed composition making it more robust. However, although the current OBR showed potential for the AD process, changes to the design are required to reduce power consumption and simplify the design before digesters based on the technology are commercially viable.

131 Figure 6.15: Multi-orifice baffle design creating the effect of 16 ‘standard design’ OBRs operated in parallel for a 200 mm diameter reactor. Reproduced (Abbott et al., 2013).

6.5 Conclusions and future work

This study has compared the performance of two digester designs based on OBR and STR technologies for AD of dairy slurry and co-digestion with glycerol. Blockages demonstrated that feed with a particulate content was not suitable for this OBR design, which required centrifugation of slurry to prevent further blockages. 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. Co-digestion with glycerol enhanced methane production by ~270% in both digesters compared to AD of dairy slurry. Maximum 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 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). Theoretical power consumption calculations for

75 mm

25 mm orifice

132 agitation were shown to be significantly less than those measured, probably due to equipment inefficiencies. These measurements also highlighted the importance of adequate equipment specification to minimise power consumption. A value of 89% less power consumption for temperature control was measured for the STR compared to the OBR. This difference was probably caused by the STR having 82% less SA:V ratio compared to the OBR, which would reduce heat loss.

These results demonstrate for the first time 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. OBRs offer a platform for the development of processes under plug flow conditions, which for AD could enable separation and optimisation of the four process stages. This is difficult to achieve with conventional STR and vessel based digesters so offers a unique aspect which needs further development to determine the extent to which OBR technology could intensify the AD process and enhance uptake of commercial AD plants. Furthermore, the baffle plates required for OBR operation provide a large, internal surface area suitable for microorganism immobilisation which, if achieved, could significantly increase the SRT and generate low feed to microorganism (F/M) ratios that increase digestion rates and methane production.

133

Chapter 7: Conclusions and future work

OBR technology provides an alternative reactor design to conventional STR, tubular and/or flat panel technologies. It could be used to intensify a wide range of processes. This could be achieved through development of continuous processes under plug flow conditions to increase throughput per reactor volume and reduce plant footprint (Stonestreet and van der Veeken, 1999, Abbott et al., 2014a, Stonestreet and Harvey, 2002); generation of intimate mixing under low shear combined with enhanced mass and heat transfer to increase reaction rates (Ni et al., 2000, Mackley and Stonestreet, 1995, Ni et al., 1995); and reductions in power consumption required for mixing to improve overall process economics (Abbott et al., 2014b, Jambi et al., 2013). This chapter presents the main findings of four research projects, followed by suggestions for future research to continue development of OBR technology towards commercial applications.