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Syngas fermentation to bioethanol 201

is continuously removed. As a result, the volume always remains constant
within the fermenter. The bacterial growth occurs mainly during the log
phase. Substrates are gone through the bioreactor continuously. The con-
tinuous fermenters allow a steady-state microbial growth if the input flow
rate is kept constant. Under steady-state conditions the microbial cell den-
sity remains constant.
Advantages: This system works all the time. As a result, labor cost
always remains low and perfect utilization of the bioreactor. The produc-
tivity is high and maintained constant product quality.
Disadvantages: The substrate must be inoculated continuously for the
continuous production of product. Sometimes, the selected microorgan-
isms are contaminated with the nonproducing microbial strain.

6.2 Fermenter for syngas fermentation

A fermenter is basically a device, in which the microorganisms are cul-
tured for the production of desired products. This system is usually
designed to give the right environment for optimal microbial cell growth
and their metabolic activity of the organism. There are various types of
fermenter that are usually used for syngas fermentation [4].

6.2.1 Continuous stirred-tank reactor
The continuous stirred-tank reactor is one of the most common fermenters
used for the estimation of key unit operation variables (Fig. 6.1). In this sys-
tem, one or more reactants (inlet syngas, nutrients) are introduced into a
reactor equipped with an agitator fixed with stirrer bars and the ultimate
products are removed continuously. The agitator rotates the stirrer bar to
ensure the perfect mixing of inlet syngas, nutrients, and the fermentation
broth uniformly throughout the whole fermentation. As a result, the com-
position of the product is uniform.

6.2.2 Bubble column reactor

The bubble column reactor (BCR) comprises a vessel containing a liquid
or a liquid solid suspension at the bottom, which has the capabilities for
the distribution of gas. The schematic diagram of the BCR is represented
in Fig. 6.2. Chemical and biochemical industries usually use this type of
fermenter, where the reactions are involving as oxidation, fermentation
chlorination, Fischer Tropsch synthesis polymerization, hydrogenation,