A comprehensive computational analysis exploring formation of caprolactam based Deep Eutectic Solvents and their applications in natural gas desulfurization
2020
Several
deep eutectic
solvents (DESs)
have been recently developed for extraction of hydrogen sulfide from
natural gas. Among these newly designed DESs, a combination of caprolactam
and tetrabutylammonium halides at a molar ratio of 1:1, as the DESs,
has the highest desulfurization efficiencies. In this work, we explored
the formation of caprolactam-based DESs using molecular dynamics (MD)
simulations and ab initio computations. The results, based on the
time average of the equilibrated production run of MD simulations,
revealed ∼15% decrease in the ionic interactions of tetrabutylammonium
halides and, more interestingly, ∼92% decrease in the hydrogen
bonds between caprolactam, thereby explaining the strong depression
in the melting point observed in experiments during the formation
of the DES. Next, simulations of the DES with mixtures of methane
and hydrogen sulfide were performed to mimic the natural gas desulfurization
process. Efficient absorption of hydrogen sulfide from natural gas
under different operating conditions (5000 and 10,000 ppm H2S, at 25 and 60 °C, and at 1 and 10 bar) can be observed from
the simulations. The results further unveiled strong interactions
between the anions of the DESs and hydrogen sulfide, with interaction
energies 10-fold higher than those of methane/hydrogen sulfide, explaining
the mechanism of desulfurization by these DESs. Additionally, two
different DESs composed of monoethanolamines/methyltriphenylphosphonium
bromide and urea/choline chloride were used to evaluate and compare
their capacity to absorb hydrogen sulfide; however, we observed that
the caprolactam-based DESs are highly efficient, particularly at low
temperatures, low pressures, and low fuel/DES mole ratios. As the
existing literature covers only experiments on the removal of hydrogen
sulfide via the DESs, the simulations presented herein provide insights
into the mechanism of absorption, thereby paving a way forward for
better design of the DESs.
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