Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 928

TABLE 18.6 Approaches to Monitoring and Control of the Chemical Environment

Approach Measuring devices and compounds monitored

pH electrode; H þ

1. Insertable probes

Redox electrodes; redox potential
Ion selective electrodes; NH 3 ,NH 4 ,Br ,Cd 2 þ ,Hg 2 þ ,Ca 2 þ ,Cl ,Cu 2 þ ,
CN , SCN ,F ,BF 4 ,I ,Pb 2 þ ,NO 3 , ClO 4 ,K þ ,Ag þ ,Na þ ,S 2 ,SO 2

4

O 2 probes (galvanic or polarographic; pO 2
CO 2 probes; activity of dissolved CO 2 , pCO 2
Fluorescence probes; NADH

Biosensors; wide range of compounds potentially detectable

2. Exit gas analyzers Paramagnetic analyzer (O 2 ); thermal conductivity or long-path infrared

analyzers (CO 2 )
Flame ionization detector; low levels of organically bound carbon,
especially useful for volatile organics such as ethanol or methanol

Mass spectrometer; O 2 ,CO 2 , volatile substances (can also be used on
liquid streams)

Semiconductor gas sensors; flammable or reducing gases or organic
vapors

3. Measurements from liquid
slipstreams

HPLC; dissolved organics, particularly useful for proteins d auto
analyzer

Mass spectrometer; dissolved compounds that can be volatilized

Enzymatic methods; potentially wide range, but glucose has received
most attention

physical environment: 1) agitator shaft power, 2) foam, 3) gas flow rate, 4) liquid flow rate,
5) liquid level, 6) viscosity, and 7) turbidity.
On-line measurements of concentrations beyond pH and DO are difficult, but progress is
being made. Table 18.6 summarizes techniques that have been considered for determining
concentration of key components.
Since many fermentation processes require extensive periods for completion, there is
a need to worry about replacement of insertable probes in addition to the probe performance.
Mechanical design may dictate placement of the probes, even though heterogeneity in a large
fermentor may be position dependent. Because of potential probe fouling, probes need to be
installed with sufficient turbulence to help keep them clean. Due to extensive periods for
fermentation completion, it is important that probe response be stable for extended periods.
Probe fouling in an extended fermentation is a significant challenge. Probe drift can be
a problem and in situ recalibration is not always possible.
Advancement in exit gas instruments at lower costs (mass spectrometer) has made
this more attractive. These devices can be installed for use on several fermentors. The