Methane is the simplest hydrocarbon and the second most-released greenhouse gas, next to carbon dioxide. Anthropogenic methane emissions have been increasing in recent decades, and the remediation or capture of such emissions has become a global concern which rises with the increase in emissions volume. Burning of this methane at spot sources of high concentration has been an attractive choice for a long time, but this does not address the emission of carbon dioxide as a result. Carbon dioxide is chiefly an undesirable byproduct of the burning of methane-rich gas streams for heat and power. This is compounded by the fact that as these methane emissions increase, so will the amount burned, and thus the amount of carbon dioxide released. A different method of utilization of methane is biological upcycling. Biological upcycling utilizes methanotrophs, which are capable of deriving energy and carbon directly from methane. This process fixes a portion of the carbon fed into cellular biomass, preventing its release as greenhouse gas. In addition to this, biological upcycling provides carbon-rich byproducts such as single-cell protein, thermoplastics, lipids, and methanol. In addition to this, the foundational research regarding methanotrophs, their products, and their properties spans the better part of the last fifty years, and they have been noted as having the potential for industrial significance. Methanotrophs are not utilized in industry despite their benefits, and this is largely due to the inefficiency with which methanotrophic bioreactors operate. Methane has very low solubility in water, and this leads to low mass transfer rates, as the concentration driving force is low. Research into methanotrophic bioreactors universally notes them as being mass transfer limited for the methane substrate. Other bioreactor systems utilize carbon sources with much greater solubilities (sugars, etc.), but methanotrophs grow on a gaseous substrate with low solubility, which leads to low productivity. Much research in the last twenty years has been directed at using various techniques to improve the mass transfer properties of methanotrophic bioreactors. Improvements in the volumetric mass transfer coefficient (kLa) of these reactors has nearly always resulted in higher cell density, better product formation, and ultimately greater remediation of the methane pollutant. Several methods for improving the mass transfer properties of methanotrophic bioreactors have been investigated, including increasing area for mass transfer, increasing solubility of the substrate, utilization of a non-aqueous phase (NAP) to improve mass transfer, and even metabolic engineering of methanotroph strains for increased utilization of the substrate. The method investigated by this study for improvements to the kLa of these bioreactors is variation in the composition of gas fed to a methanotrophic bioreactor, and the utilization of an oil phase to improve mass transfer. Bioreactors utilizing NAP to improve mass transfer are called two-phase partitioning bioreactors (TPPBs). TPPB experimentation has generally revolved around utilizing an existing bioreactor or bioreactor design, and simply ‘modifying’ it by adding a small amount of an oil phase (10% by volume, usually). These studies are generally performed in stirred tank reactors, but some studies on bubble columns exist. Bubble columns are an interesting avenue of research for TPPBs, as they offer increased gas contact time, and avoid harmful shear rates inherent in stirred reactors. Additionally, bubble columns are very simple to construct and operate. Bubble columns will also benefit the most from the TPPB modification, as mass transfer rates are generally low. This research investigated the effect of oil selection and recirculation on the kLa of a bubble column TPPB, and additionally investigated the effect of gas composition, which was performed to determine the feed gas compositions which hindered the kLa of same. This study aims to determine the effectiveness of various oil additives, as no study has yet compared the primary oil-phase additions directly in the same experimental setup, nor has much investigation of novel modifications to this design been researched. This study hypothesized that the recirculation of the oil phase would have a beneficial effect on the kLa that goes beyond simple addition, and that this improvement would follow the overall solubility of methane in the
oil phase (that is, the improvement would be directly related to the increasing solubility). It was hypothesized that for the gas composition testing that the kLa for both methane and oxygen would be improved by increasing the balance of gas until it reached the saturation concentration, and that there would be limitations for methane kLa at high air composition, and vice versa. Experimentally, these studies were performed by determining the kLa of the bioreactor without any oil addition, and then testing with simple addition of various oil phases (silicone oil, paraffin oil, and mineral oil). Subsequently, testing of the mass transfer coefficient was performed with a recirculation of the oil phase, which consisted of a pumped loop which collected the oil at the top of the reactor and fed it into a modified sparger at the bottom, which was hypothesized to improve the contact between oil and water, and thus improve the mass transfer rate. For the gas composition experiments, 1 LPM total gas was introduced to the reactor by an air stone sparger at various ratios of air and methane, and the kLa measured for both methane and oxygen. The methane/oxygen kLa was measured at the extrema (>10% v/v for each gas) to determine limiting concentration for kLa. In all cases, the utilization of an oil phase improved the mass transfer coefficient significantly, which agrees with two-phase partitioning bioreactor research. Naturally, for the gas composition testing, there was no kLa observed for oxygen at 100% methane flowrate, and vice versa; however a dependence on composition was observed in the kLa for these experiments. In addition to this, it was observed that the recirculation of the oil phase offered a minor improvement in the mass transfer coefficient when compared to simple oil addition. One element of this study not tested was the effect of density on the mass transfer coefficient, as the differences between them were judged to be small. Improvements to the methane gas transfer coefficient and innovations in process design will drive methanotrophic bioreactors into an industrially-significant place, and allow them to produce useful products while simultaneously remediating methane emissions, all with less carbon dioxide release than simple burning.
