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Syngas fermentation to bioethanol 203

wastewater treatment, etc. This fermenter is used because of its excellent
heat and mass-transfer characteristics, less operational cost, and durability of
anytypeofsolid material [42]. In addition, this type of fermenter has the
facilities for the addition or removal of any catalyst from the column.
Nevertheless, there are some limitations or challenges that are associated
with this fermenter. The syngas is fed to the column through the gas sparger
at the bottom of the column. The gas in the column may form separate
bubbles that rise and spread out due to their buoyancy. It also induces mix-
ing of the continuous liquid phase, as reported by Holland [42].

6.2.3 Monolithic biofilm reactor
Monolithic biofilm reactor (MBR) is a type of fermenter integrating a
cordierite monolithic packing material within a bubble column. It offers a
perfect way to sustain the high cell density systems due to its high mass-
transfer capacity. The monolithic column is fixed inside the plexiglass col-
umn by two symmetrical block rings located at the top and bottom of the
monolithic column. The heat loss is accurately minimized by covering the
monolithic column with an insulation sheet. The medium is also perfectly
circulated between the column and the vessel during syngas fermentation.
The disadvantage of the fermenter is clogging due to biofilm formation
during the biological process. The schematic diagram of the MBR is
shown in Fig. 6.3. In addition, a BCR is established as a control to gauge
the mass transfer and the fermentation of syngas. The parameters of the
column and the operational conditions were identical to MBR reported
by Shen et al. [43].

6.2.4 Trickle-bed reactor
The trickle-bed reactor is one type of chemical reactor which involved
the downward movement of a liquid and the downward or upward
movement of syngas over a catalytical packed bed (Fig. 6.4). This type of
fermenter is intrinsically nonstop in operation. Due to the low operating
cost and nominal plug flow of both gas and liquid phases, a good control
occurs over the process. As a result, it maintains good product quality. It
has the facilities of random packed catalytical bed. Therefore it has the
ability to inherently a single production plant or single catalytical plant.
On the contrary, it has no heat transfer facilities within the bed. Thus the
designs for trickle bed and batch plants are based on various flow rates,
design specification, and cost extrapolations.