Fundamental understanding of optimal methods of contacting ionic liquids and gases at high flow rates with a focus on capturing fugitive methane emissions from mining operations

Concerns over greenhouse gas (GHG) emissions (particularly methane) due to coal mining operations have resulted in the development of a suite of mitigation technologies in the past decade. The majority of these technology platforms focus on abating the so-called ventilation air methane (VAM) which is essentially the emissions of methane from the mine’s ventilation shaft(s). The current VAM abatement technologies, like oxidation processes, operate at temperatures above the auto-ignition temperature of methane and, hence, increase the risk of accidental fire and/or explosions in the mine. Development of technology platforms that can be operated near the ambient temperature can eliminate the risk of such fires and explosions. One such technology platform could be gas absorption process that normally operates at a temperature much lower than the auto-ignition temperature of methane. A technology like this can separate methane from VAM at a temperature close to ambient and, thus, can be considered as an effective alternative to current VAM abatement technologies. One of the important factors that significantly affects the gas separation performance of an absorption technique for VAM abatement is the choice of a liquid solvent with high methane removal efficiency. Moreover, an environmentally friendly solvent can contribute to reducing the negative environmental impact of the absorption technique for VAM mitigation. In recent years, ionic liquids have received much attention as green solvents with many advantages including high thermal stability and low vapour pressure that make the liquids an environmentally attractive alternative to the conventional solvents. Based on the challenges of utilising an absorption technique for removing methane from VAM stream using ionic liquids like high flow rate of VAM stream, gas flow rate fluctuation, low methane concentration, and high viscosity of ionic liquid solvent, it seems that the conventional packed bed column system is a more suitable contactor comparing to other absorption technologies like venturi scrubber, bubble column, microfluidic contactor, and spray tower systems. Hence, in this research, the packed bed column absorption technique was chosen as a promising method for methane capture from VAM using the ionic liquid solvent. In this research, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][TF2N]) was selected to be applied in the packed bed column absorption process based on its physical properties. According to the literature review, Henry’s constant (MPa) of [BMIM][TF2N] is lower than most of the investigated ionic liquids making it a promising solvent for methane absorption. Moreover, the calculated packing wetted surface area of [BMIM][TF2N] was greater than other studied ILs due to its lower viscosity and surface tension. Therefore, despite the lower methane to nitrogen selectivity of [BMIM][TF2N] compared to the studied ionic liquids in this study, [BMIM][TF2N] was chosen as a suitable ionic liquid for methane capture using packed bed column absorption technique because of its promising characteristics like high methane solubility and wetted surface area of packing. This research aims to investigate and optimise methane capture from the simulated VAM stream in a packed bed column using [BMIM][TF2N] experimentally and mathematically to evaluate the suitability of this process for separating methane from VAM. The objectives of this work included experimental investigation of the impact of absorption operating conditions (e.g., temperature, gas flow rate, liquid flow rate, packing size) and desorption operating conditions (e.g., temperature, vacuum pressure) on the methane removal efficiency, and nitrogen removal efficiency as a function of liquid flow rate. The other objectives of this research were conducting the mathematical study to investigate the methane removal efficiencies of different ionic liquids with insight into viscosity, surface tension, and Henry’s constant of the ionic liquids and wetted surface area of packing, and the effect of packing material on the methane removal efficiency as a parameter that was not experimentally investigated. After that, the model was optimised to find the optimum operating conditions. To achieve the aims of the research, an experimental setup was fabricated consisted of the absorption and desorption units that methane was removed from the flow gas using [BMIM][TF2N] inside the packed bed column, and then ionic liquid was regenerated inside the regeneration vessel to provide fresh ionic liquid for the absorption process continuously. Moreover, the experiments were conducted at different operating conditions to evaluate the process performance. A mathematical modelling was also carried out and the model was validated by the experimental results that the modelling and experimental results were in a good agreement. Based on the experimental results, at 1 vol% inlet methane concentration to the absorption unit, this process could concentrate methane in the regeneration vessel up to 4 times greater than inlet methane concentration to the process. Moreover, increasing the gas flow rate from 0.1 to 0.6 L/min at constant liquid flow rate of 0.1 L/min resulted in decreasing the methane removal efficiency because of the short contact time between gas and liquid phases. Although increasing the gas flow rate contributed to decreasing the mass transfer resistance in the gas phase, the negative effect of decreasing the contact time was more significant because the mass transfer resistance in the gas phase is negligible compared to the liquid phase. The maximum methane and nitrogen removal efficiencies were observed at the maximum studied liquid flow rate (0.5 L/min) due to the positive effect of increasing liquid flow rate on liquid phase turbulence and the mass transfer coefficient. At a liquid flow rate of 0.5 L/min, the nitrogen removal efficiency was approximately 5 times greater than that of CH4 due to the lower Henry’s coefficient (MPa) of methane in [BMIM][TF2N] compared to nitrogen. Furthermore, rising the absorption temperature had negative effect on methane removal efficiency. High temperature resulted in low mass transfer as solubility decreased with temperature even though increasing the temperature reduced the viscosity. The trade-off between solubility and viscosity occurred at relatively low temperatures. Regarding the flow pattern, the methane removal efficiency achieved by counter-current flow pattern was greater than that of the co-current flow pattern due to the greater average mass transfer driving force between gas and liquid phases in the counter-current flow compared to the co-current flow pattern. However, the effect of flow pattern on the methane removal efficiency was lower than the other operating parameters. The packing size had a significant impact on methane removal efficiency. Reducing packing size, from 10 mm to 6 mm, contributed to improving gas and liquid phases contact surface area that led the methane removal efficiency to increase. Although decreasing the packing size had a negative influence on the Reynolds number and liquid phase turbulence due to reducing the equivalent particle diameter of the packing, the positive impact of increasing the packing surface area on the mass transfer rate was more significant. The desorption operating conditions including desorption temperature and vacuum pressure affects the methane removal efficiency. Reducing pressure inside the regeneration vessel up to -0.015 MPa improved the methane removal efficiency by about 21%, and methane removal efficiency increased by around 23% by increasing the desorption temperature from 353.15 to 433.15 K for inlet methane concentration of 1 vol%. Although desorption temperature affects the CH4 removal efficiency, the rate of increasing the methane removal efficiency graph decreased by increasing desorption temperature so that at temperatures greater than 403.15 K the methane removal efficiency did not change significantly. Based on the experimental results, when the desorption temperature and vacuum pressure of 403.15 K and -0.01 MPa were considered as a base case, the operating parameters of the absorption unit including gas and liquid flow rates, absorption temperature and packing size had more significant influence on the methane removal efficiency comparing to the desorption unit operating parameters such as temperature and vacuum pressure. Among the absorption unit operating parameters, the packing size had the greatest influence on the methane removal efficiency. Decreasing the packing diameter up to 50% compared to the base case of 10 mm packing size resulted in enhancing the methane removal efficiency by about 50%. It was found from the experimental data optimisation results that the maximum methane removal efficiency at the minimum studied packing size could be obtained at the minimum studied absorption temperature and gas flow rate and the maximum studied liquid flow rate. According to the modelling results, the following trend for CH4 removal efficiency of different studied ionic liquids was achieved: [BMIM][TF2N]˃[EMIM][TF2N]˃[HMIM][TF2N]˃[EMIM][BF4]˃[BMIM][BF4]˃-[BMIM][PF6] due to the lower Henry’s constant (MPa) and greater packing wetted surface area of [BMIM][TF2N] compared to the other studied ionic liquids. Moreover, the effect of the packing material was investigated by modelling, and the trend of methane removal efficiency was as follows: Glass˃Steel˃Carbon˃Ceramic˃PVC˃Paraffin. The reason was that the critical surface tension of glass is greater than other packing materials that led the greater glass packing wettability. According to the industrial scale optimisation results, the methane removal efficiency of 96% could be obtained by the absorption process including three columns with column height and diameter of 10 m and 4 m, respectively, and packing diameter of 6 mm when the ga