Development of a direct monitoring approach of CO/H₂ in bioreactors for syngas to biofuels conversion

Abstract

Syngas fermentation is a novel technology to produce biofuels from syngas using acetogens. Precise monitoring of dissolved CO/H₂ concentrations helps to maintain the proper activities of the acetogens and is of great importance for a high product yield and operation stability. A new approach was developed to measure the dissolved CO/H₂ concentration with optical sensing and electrochemical methods, respectively.

An indirect mid-infrared measurement system was developed to measure dissolved CO concentration. The measurement system included a gas extraction system for dissolved CO degasification and a non-dispersive mid-infrared CO sensor for gas phase CO concentration measurement. The gas extraction system used a hollow fiber membrane contactor to extract dissolved CO in a fixed volume of liquid sample. A vacuum was applied to the contactor to accelerate the degasification. The non-dispersive mid-infrared CO sensor used a room-temperature, mid-infrared light emitting diode as a mid-infrared source. The sensor was designed to detect CO absorption in the mid-infrared range between 4.7 µm and 4.9 µm as this range has minimal spectral interference from other chemicals in the syngas fermentation. The infrared signal was detected with a thermoelectric cooled photodetector. A digital lock-in amplifier was designed to improve the quality of the collected signals from the preamplifier.

A commercial electrochemical dissolved H₂ sensor was introduced to measure the dissolved H₂ concentration. An integrated dissolved CO/H₂ measurement system was built by integrating the dissolved H₂ sensor to the developed dissolved CO measurement system. Performance evaluation of the integrated measurement system showed a maximum mean absolute percentage error of 10.83% and a root-mean-square error of 1.40 mg/L for the dissolved CO concentration measurement. The measurement on the dissolved H₂ concentration measurement was affected by the supersaturated dissolved H₂.

The developed integrated measurement system provides a new method that can measure the dissolved CO/H₂ concentrations in the syngas fermentation medium. The method would be helpful to future optimization and commercialization of the syngas fermentation technology.