Shown here are two sets of analyzers for measuring carbon dioxide and oxygen in fermentation exhaust gas (off gas). They are crucial for monitoring oxygen uptake rate (OUR), carbon dioxide evolution rate (CER), and respiratory quotient (RQ)(see equations below). These three parameters can provide important information about rates of biochemical reactions in many fermentations and the extent to which they have occurred. They have been known to indicate levels of product formation, substrate utilization, problems with medium formulations as well as growth rate and metabolic states of the microorganism in the fermenter.
The two orange colored analyzers built by Infrared Industries, Inc. measure CO2 in the ranges of 0-6% and 0-20% using infrared light, while the two grey units built by Beckman Instruments, Inc. are paramagnetic O2 analyzers which can measure oxygen in the ranges of 0-5%, 0-10%, 0-25%, and 0-50% of off gas.
The numerous ranges are needed because, depending on the microorganism and the medium it is being grown in, it may produce only a trace of CO2 and use only a trace of O2 or it may produce >15% CO2 and utilize 45% of the available oxygen. These ranges are automatically switched as needed by the process control computer.
Calibration is also done by computer and timing circuits. First pure Nitrogen is allowed to flow through the analyzers for 15 minutes to flush them clear of CO2 and O2, then data points are taken and averaged for 5 minutes with this "zeroing gas". Then the computer activates valves to allow an "upscale" calibration gas of know concentration to flush them for 15 minutes after which time data points are collected for 5 minutes to get averaged upscale values (14, 26, 73, 87, 136, 199, 226).
NOTE: OUR, CER, and RQ can be determined continuously, on-line with CO2 AND O2 analyzers and computer using the equations:
OUR (mmoles/l.h) = FN / VL (( PO[ in ] / Pt - PO[ in ] - PW[ in ] - PC[ in ] ) _ ( PO[out] / Pt - PO[out] - PW[out] - PC[out] ))
CER (mmoles/l.h) = FN / VL (( PC[out] / Pt - PC[out] - PW[out] - PO[out] ) _ ( PC[ in ] / Pt - PC[ in ] - PW[ in ] - PO[ in ] ))
RQ = CER / OUR
In this view are two continuous fermenter exhaust gas drying systems. Fermenter exhaust gas (off gas) is saturated with water vapor and must have all water removed prior to entering the CO2 and O2 analyzers described above (humidity will give false readings and eventually destroy them)!
In farthest upper right corner, mounted on the wall, are two countercurrent gas dryers made by Perma Pure Products, Inc. Fermenter off gas passes through the center of these dryers which are comprised of multiple tubes ~50, permeable to water. Around these 50 tiny tubes flows, countercurrently, a very dry gas (Nitrogen, purified) which absorbs water, as it passes through the walls of these internal tubes. The result is, with slight differential pressure, water is pulled out of the fermenter off gas and into the drying gas. These gas dryers are very effective in removing 99% water vapor from fermenter off gas. The remaining 1% is removed by the blue Drierite Canisters, also mounted on the wall (68, 87, 195).
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