Biomedical Engineering Reference
In-Depth Information
Similarly, Richardson et al. (2012b) compared the performance of the raceway,
horizontal tubular reactor, and airlift tubular reactor as a component of the algal bio-
refinery producing biodiesel and biogas. Their comparison was made using literature
data for Phaeodactylum tricornutum , extensively studied in the Aquatic Species
Programme (Sheehan et  al., 1998). Considering an integrated biorefinery system
with combined biogas production, the relative NER values for these photobioreac-
tors compared to the raceway were 64% and 8%, respectively. Under the operating
conditions selected, the NER of the airlift reactor was unacceptable, owing to the
low productivity achieved relative to the energy input. The airlift reactor can be
operated at much reduced gas flow rates and concomitant reduction in energy input
without compromising productivity (data not shown), indicating the need to make
these comparisons using optimized performance data relevant to commercial-scale
operation. In all cases, the reactor energy requirement dominated that of the process,
with that of the horizontal tubular reactor being some twofold that of the raceway
per unit biodiesel. Owing to the lower biomass and oil concentrations achieved in the
raceway reactor, this advantage of reduced reactor energy was partially offset by the
greater pumping energy required for the larger volume processed from the raceway
(2.2-fold); the pumping energy within the raceway biorefinery is a quarter of the
reactor energy requirement. Extending the analysis beyond energy, the acidification
and eutrophication impacts of the horizontal tubular reactor were 61% and 73%,
respectively, of the raceway system under the standard operating conditions selected.
The GWP was negative for the raceway system compared to a positive value of 60%
for the horizontal tubular reactor.
Recognizing the increased energy requirement of traditional photobioreactors for
mixing and mass transfer as well as manufacture, Batan et  al. (2010) assessed the
sparged polyethylene photobioreactor bags. While they report positive NER values for
these systems, agreement in the literature with respect to the feasibility of the airlift
system for biofuel production has not been found and further assessment is required.
Razon and Tan (2011) assessed the combined production of biodiesel and biogas
using Haematococcus pluvialis . While low biomass concentrations and productivity
resulted in a negative NER, the energy requirement of the flat-plate bioreactor used
for intermittent inoculum supply exceeded that of the raceway system used for large-
scale production by some twofold, thus confirming the much higher energy demand
per unit biofuel in closed reactor systems.
Stephenson et al. (2010) disaggregated the contributions to the reactor energy and
GWP in the tubular airlift reactor and raceway pond. These are shown relatively in
Figure  9.2 (see color insert), where the total fossil energy requirements estimated
for cultivation in the tubular airlift reactor and raceway under standard conditions
were approximately 230  and 29  GJ per tonne biodiesel formed, respectively. The
corresponding GWPs were 13,550  and 1,900  kg CO 2   per tonne biodiesel, respec-
tively. In addition to the magnitude, the relative values illustrate that electrical energy
for mixing and mass transfer dominates the reactor energy and related GWP in the
tubular reactor. The lower mixing energy in the raceway results in the energy for com-
bined nitrogen provision being significant. Furthermore, the much larger raceways
required, owing to lower productivities achieved, lead to the construction compo-
nents making a more dominant contribution, especially the PVC liners of the ponds.
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