Biomedical Engineering Reference
In-Depth Information
In most cases, such instruments are shared among several reactors. This is
accomplished by multiplexing the analyzer's inlet using revolving selection valves
connected to the individual exhaust gases. Up to 256 ports have been reported. It is
necessary to purge theses lines well, in order to avoid long dead and delay times.
A more complete survey of methods, techniques, and instrumentation is given by
Pollard and Christensen [
25
].
Several interesting variables—from a technical as well as from a physiological
point of view—can be derived from exhaust gas analyses. Most important is the
physiological variable RQ, the respiratory quotient. It is defined as the quotient of
CO
2
production rate over O
2
consumption rate in units of mol mol
-1
and can be
directly derived from gas composition data. In a first approximation, the RQ is the
negative slope of the line connecting data points in a phase-plane plot of CO
2
content versus O
2
content (Fig.
1
).
Strictly, this is true only if the RQ equals unity. If the RQ deviates from 1, the
gas mass flow rates of fresh and exhaust gas differ; rather than measuring each flow
rate individually, one can assume that all gas components except O
2
and CO
2
are
inert (if appropriate) and calculate the exhaust gas flux from the respective inert
gas balance and, after rearrangement, the RQ as
RQ
¼
y
out
CO
2
y
i
CO
2
y
i
O
2
y
out
CO
2
þ
y
out
O
2
y
i
CO
2
;
y
i
O
2
y
out
O
2
y
i
O
2
y
out
CO
2
þ
y
out
y
i
CO
2
O
2
where y denotes the molar fractions of O
2
and CO
2
in the fresh (superscript ''in'')
and exhaust (superscript ''out'') gas, respectively. Care must be taken that the gas
analyzers are well calibrated because, otherwise, the quadratic terms (coming from
the inert gas balance) can lead to dramatic error propagation. If gases other than O
2
and CO
2
are involved in the bioreaction, the term ''inert'' needs to be redefined
appropriately of course.
Yet another error can be caused by chemisorption of CO
2
to HCO
3
-
or even
CO
3
2-
, at neutral pH but specifically under alkaline process conditions. This
reaction is almost negligible under acidic conditions, e.g., for cultivation of yeasts,
fungi or lactic acid bacteria.
Knowing one gas mass flow rate explicitly, one can calculate the oxygen
consumption (or uptake) rate (OUR) and the carbon dioxide production rate (CPR)
of the entire plant from the gas analysis data. Further knowing also the working
volume of the liquid phase, the respective gas transfer rates can be deduced and,
knowing the biomass concentration as well, the specific gas turnover rates (q
O2
and
q
CO2
) are easily calculated.
3 State-of-the-Art: Biomass, Substrates, and Products
Although the biomass is the (most) crucial variable in bioprocesses, online moni-
toring does not seem to be accepted as a general standard. The problem is posed by
the interferences that are captured by many online techniques. The respective results
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