3.4 Membrane bioreactors and continuous processes
Several research groups have assembled membrane bioreactor systems for contin-
uous steady-state lipolysis. For a typical process (depicted in Figure 5), the effluent
from a tank reactor is fed to an ultrafiltration membrane module. Molecules of pro-
duct (FFA), mono-, di-, tri-glycerides, and solvent are sufficiently small to permeate
the membrane; however, w/o-MEs in theory are too large to permeate. The flow of
nonpermeable materials, or retentate, is recycled directly back into the reactor. The
permeate can be collected as product and/or recycled back to the reactor. In addition
to continuous operation, a second advantage of this system is thein situremoval of
FFA, a competitive inhibitor, suggesting an increase in the rate and extent of reaction
(Chiang and Tsai, 1992b). Such an increase of rate for bioreactors has been reported
(Prazeres et al., 1993c). However, in general, bioreactor performance did not meet
expectations, a major problem of all bioreactor systems being the significant permea-
tion of w/o-MEs (the surfactant AOTand water) (Chiang and Tsai, 1992a; Prazeres et
al., 1993a; 1994). This occurrence leads to complications in downstream product
recovery and the need to replenish the w/o-ME supply in the reactor to maintain
steady state. In addition, a significant amount of the lipase and surfactant adsorbed
to the membrane surface, leading to a decreased permeation flux (Prazeres et al.,
1993a; Nakamura and Hakoda, 1995). Furthermore, a trade-off existed between de-
gree of conversion and productivity (moles of FFA generated per time per mass of
enzyme) (Prazeres et al., 1994). A second trade-off existed for w/o. A larger w/o-ME
size generally decreased the permeability of w/o-MEs (Nakamura and Hakoda,
1995), though not always (Prazeres et al., 1993a), but led to an increase in the
rate of inactivation, in agreement with the material presented in Section 3.2.5 (Na-
kamura and Hakoda, 1995; Hakoda et al., 1996a). Furthermore, although FFA per-
meated through the membrane, its concentration in the retentate was greater than or
equal to its permeate concentration, indicating a strong degree of association of FFA
molecules with the w/o-ME interface, as discussed in Section 3.2.2, hence leading to
product inhibition (Prazeres et al., 1993c).
3.4 Membrane bioreactors and continuous processes 59
Figure 5. Membrane reactor scheme for w/o-ME systems.