Selective Absorption of Methane by Ionic Liquids (SAMIL)

This third and final stage of this project was the culmination of a multi-phase program of work investigating the applicability of ionic liquids for the selective absorption of methane from ventilation air. The program of work consisted of three phases:

• C27008 Selective absorption of methane by ionic liquids (SAMIL).
• C28076 Selective absorption of methane by ionic liquids (SAMIL). Phase 2: demonstration in a packed bed.
• C28076 (extension) Selective absorption of methane by ionic liquids (SAMIL). Phase 3: Pilot-scale demonstration and mine site integration study.

Proposed as an alternative approach to high temperature oxidation of ventilation air methane (VAM), the separation of methane using ionic liquids would occur at temperatures significantly less than the autoignition temperature of lean air-methane mixtures. This approach has the potential to remove the safety concerns related to connecting high temperature abatement processes to the ventilation system of an underground coal mine. The captured methane could be utilised to produce electricity and/or heat for the process. A key requirement for any VAM abatement process is to minimise the energy demand for the project, with a target of supplying all power requirements from the VAM.

Phase 1, C27008,  of the project focussed on fundamental investigations into the chemistry of ionic liquids and low-concentration methane mixtures. A reversible process for the selective absorption of methane in ionic liquids was successfully demonstrated in Phase 1. The process occurred at less than 100 ^oC, significantly less than the autoignition temperature of lean air-methane mixtures. Selectivities of methane were greater than ideal selectivities predicted by individual species absorption data reported in the literature.

Translating the fundamental findings from Phase 1 to a suitable absorption process was the focus of Phase 2. The high viscosity of ionic liquids required a novel absorption process that could facilitate a high gas-liquid mass transfer rate. A rotating packed bed experimental apparatus was constructed to investigate the influence of various process parameters such as absorption temperature, residence time, contact surface area and rotational speed on the absorption of methane. Optimisation of the ionic liquids was not included as part of the project and the best performing ionic liquid from Phase 1 of the project was selected for Phase 2. The experimental results confirmed that a rotating packed bed was a suitable technology for the process and the process could be undertaken at low temperatures and pressures.

The experiments in Phase 2 were conducted as batch absorption and desorption experiments. A continuous process would be required for a mine site. Phase 3 of the project consisted of demonstrating the process in a continuous bench-scale pilot-plant with a focus on determining the scale and power consumption of a process for a 20 m³/s module at an operating mine site.

METHODOLOGY

To demonstrate the continuous operation of a bench-scale pilot-plant and understand the mine site integration challenges of the ionic liquid process, the following tasks were undertaken:

• Design and construction of a continuous bench-scale pilot-plant;
• An experimental program investigating key factors that influence the absorption of gases by ionic liquids such as gas and liquid flow rates;
• Process modelling to determine the energy requirements of a 20 m3/s process;
• Evaluation of the process for application at a typical mine site.

The key findings for the project were:

• From a technical standpoint the process was successfully demonstrated in a continuous absorption and desorption process.
• Variability of VAM concentration will have little influence on the performance of the process.
• The ratio between the gas flow rate and the ionic liquid flow rate is the key parameter influencing the performance of the process.
• The rotating packed bed process was more efficient than a conventional absorption column.
• Mine site integration of the technology would be unviable without significant improvements to ionic liquid properties.

CONCLUSION

While the program of work successfully demonstrated the selective absorption of methane using ionic liquids, the data obtained from the continuous operation of the process indicated that the process would not be viable for large-scale applications. Without significant improvements in ionic liquid properties (through discovering new ionic liquids), further examination of VAM capture using ionic liquids is not warranted.

The reports for C28076 (published May 2023 and August 2020) are provided as a set.