Novel absorption technology of monoethanolamine using ionic liquid as phase separator

Ionic liquid biphasic solvent for low energy consumption biogas upgrade

The depletion of fossil fuels has put great pressure on energy utilization and the environment, so the development and utilization of renewable energy needs to be increased. Biogas is a green, safe and efficient renewable energy source, but a large amount of carbon dioxide reduces its calorific value, so CO 2 capture is required. Amine washing technology is a relatively mature CO 2 capture technology, which is suitable for large-scale biogas transformation, but its high renewable energy consumption limits its commercial application. Ionic liquids (ILs), as a new type of solvent, can capture CO 2, but high viscosity will affect mass transfer. At present, people often add a small amount of water to improve its high viscosity phenomenon.

Under the action of the phase change absorber, the rich amine solution after biogas transformation can be separated into the rich liquid phase and the poor liquid phase, and then the biogas upgrade treatment can be achieved by regeneration and mixing treatment, and the desorption energy consumption is reduced. ILs are less studied as phase splitters, but adjusting their addition amount can achieve phase separation, reduce energy loss and improve the thermal stability of the absorber, which brings research potential for amine-ILs-water three-phase mixtures. 1-butyl-3-methylimidazolium tetrafluoroborate (BF) is widely used in the CO2 capture process due to its good thermal stability, and can improve the permeability and selectivity of CO2.

Based on this, Jiang Jianguo's research team from Tsinghua University developed an aqueous two-phase solvent composed of monoethanolamine (MEA) and BF for CO 2 capture in low-energy biomethane. Through experimental research on various biogas upgrade characteristics and calculation and prediction of physical parameters, the basic physical parameters of the mixed system, biogas upgrade effect and phase separation characteristics were comprehensively studied, and the influence of ionic liquids on the mass transfer process was discussed.

Experimental results and performance analysis

The researchers investigated the effect of MEA/BF aqueous phase mixtures with different mass concentrations on biogas upgrading and regeneration. The results showed that the amount of BF added had little effect on the purity of biogas methane after upgrading, and the purity that met the requirements could be achieved, and the effect was best at 40 wt%. The density and viscosity of the mixed solution showed an upward trend with the increase of mass concentration, but when the BF mass fraction was below 50 wt%, the viscosity could be maintained at a lower level. However, when the BF mass fraction exceeded 60%, the viscosity would increase sharply and was not suitable as a separator. The volume of the rich phase after cleavage of 30MEA-40BF aqueous two-phase solvent accounts for about 50% of the total volume, and almost all of the CO2 is enriched in the rich phase, which is beneficial to the cycle process, and the energy consumption is 43.19% lower than that of 30 wt% MEA.

Reaction mechanism and intermolecular interactions

The researchers analyzed the reaction mechanism and intermolecular interactions in the process of biogas upgrading in a two-phase solution. It can be known that MEA, protonated amine, carbamate and carbonate/bicarbonate are located in the rich phase, and BF is located in the poor phase. With the increase of CO2 absorption, the relative density of the two phases changes, and the product has a tendency to dissolve in water. The three-dimensional visualization study conducted by Gaussian quantum computing combined with Multiwfn and VMD shows that the solvent effect of the ionic liquid BF reduces the reaction energy barrier and accelerates the absorption process. The interaction between molecules is mainly caused by van der Waals forces and hydrogen bonds, and the ion pair has a strong van der Waals effect within a certain range. The amount of reaction products produced will determine whether the positions of the poor and rich phases are flipped.

Phase separation kinetics

Finally, the researchers studied the phase separation kinetics of the two phases under different CO _ 2 loads, and found that with the increase of the initial CO _ 2 load, the stabilization time was shortened, the volume of the rich phase was maintained at about 50%, and the carbon enrichment rate of the two-phase agent could reach more than 95% when the CO _ 2 load was 0.5 mol CO _ 2/mol amine, and the phase separation efficiency was greatly improved. The aqueous solution containing carbamate gradually moved down, while the ILs carried part of the water up, resulting in phase separation. During the separation process, the rich phase was mainly affected by gravity and intermolecular forces, and the poor phase was mainly affected by resistance and buoyancy.

Conclusion

In summary, the MEA-BF aqueous phase mixture system can make up for the shortcomings of both amines and ionic liquids while maintaining the advantages of the two, and achieve rapid capture of CO2, high purity preparation of biomethane, and low energy regeneration and recycling of ionic liquids in the process of biogas upgrading.