(375f) Construction of a Sensor for Rapid and Online Measurement of Dissolved Methane in Bioreactor Systems

Two phase partitioning bioreactors (TPPBs) have seen a rapid increase in study over the past several years due to their usage with methanotrophs to remediate methane emissions and generate useful products. In order to use these reactors effectively, the ability of the reactor to promote the gas transfer of methane into the liquid phase must be independently determined, which is something that few studies have tackled in recent years. The objective of this study was to construct and test a low-cost and easily implemented dissolved methane sensor for the estimation of the gas transfer coefficient of methane in TPPB reactors, which have been shown to selectively promote methane transfer into the media. Much of the research in the field currently relies on correlations between the oxygen transfer (a much easier measurement to take) and the methane transfer, which is simply not viable for TPPB reactors. This study is based on previous work [1] to independently measure dissolved methane, and expands on the capabilities for integration and data collection, while providing more knowledge and plans for future researchers. The principle of operation is that methane can preferentially dissolve through a thin silicone membrane between liquid and gas, causing dissolved methane to migrate into the gas phase on the opposite side of the membrane. If the other side of the membrane is flushed with a purge gas, this dissolution may be promoted by maximizing the concentration gradient between the two phases. The gas may then be passed through a traditional gas-phase methane sensor (which are highly selective, sensitive, and cheap) for determination of the liquid-phase dissolved methane concentration. The rate of diffusion across this membrane (and thus the concentration in the purge gas) is correlated to the concentration The silicone tube used for testing was 10cm in length, with a 0.25 cm and 0.35 cm inside and outside diameter respectively (Detakta Co. PN: 02502). The length of the tube was 10cm Based on previous studies, the nitrogen purge gas flowrate was specified at 50 mL/min. This flowrate provides the best combination of sensitivity (negatively impacted by increasing gas flowrate) and rapidity (positively impacted by same). A housing was designed in Fusion 360 and 3D printed to direct the purge gas flow into the mesh dome of the sensing circuitry. The primary objective of this study is to produce an easily replicated sensor for measuring the volumetric mass transfer rate of methane gas, which is an important factor in bioreactor operation, design, and scale-up. Measurements of this parameter were performed using the gassing-out method, in which the sensor is acclimated with a purge of pure nitrogen (to remove any trace methane), and then sparged with methane (in this case, 100% methane gas at 2 L/min). The response of the sensor as it reaches a new equilibrium at saturation is linearized to yield the value of the transfer coefficient. The response over time of the sensor is demonstrative of its effectiveness even under changing gas conditions. For instance, it appears to have (as stated) a high selectivity for hydrocarbon gases, with no change in the baseline reading of the sensor when nitrogen, air, or helium are passed through the liquid phase. The two phase partitioning bioreactor (TPPB) used for the evaluation of the sensor had a volumetric mass transfer coefficient of 37.8±2.2 hr-1, which is much higher than that of previous studies, which show around 15 hr-1 [2]. This is credited to the oil circulation of the bioreactor, which promotes the gas transfer from the methane gas bubbles to the aqueous phase. Little concrete research exists, however, for the measurement of the methane transfer coefficient in bioreactors. This is likely due to the difficulty in constructing such a sensor, and many works use a correlation to relate the transfer rate to that of oxygen, a much easier measurement to achieve. This construction and validation lays the groundwork for an independently calculated methane transfer coefficient, a necessary element for the progression of methanotrophic bioreactor research, especially for TPPB style reactors.