Hydrate-based CO2 capture and CH4 purification from simulated biogas with synergic additives based on gas solvent

Abstract

Treatment and subsequent use of biogas are garnering huge interest for both energy recovery and mitigation of environmental impact. However, separation process is pivotal for increasing its calorific value and removing CO2. This work presents the kinetic and separation efficiency study as well as microcosmic structure analysis for purifying simulated biogas (45.0mol% CO2/CH4 binary mixture) through hydrate crystallization approach. Particularly, synergic additives comprise gas solvent (dimethyl sulfoxide (DMSO)) and traditional hydrate promoter (tetrahydrofuran (THF) or tetra-n-butyl ammonium bromide (TBAB)) were proposed to enhance the hydrate-based separation process. The promotion mechanism was explored through in-situ Raman spectroscopy. The residual gas phase and the decomposition gas phase from the hydrate slurry were sampled and analyzed. Based on the experimental data, the gas storage capacity, unit system gas consumed rate, gas selectivity and separation efficiency were calculated for evaluating the separation process. It was found that, the synergic additives could promote the mixture hydrate formation process due to DMSO (acid gas solvent) could improve both rate and selectivity of CO2 during the dissolution and diffusion processes. In addition, the Raman analysis reveals that the simulated biogas forms structure II hydrate and semiclathrate framework with THF–DMSO and TBAB–DMSO respectively, and CH4 molecules are only found in the smaller (512) cages of the mixture hydrates. It is inferred that DMSO just performs as an acid gas solvent during the gas dissolution and diffusion processes but not participate in the hydrate framework formation. It will be of practical interest in relation to resolving the bottleneck of hydrate-based biogas purification technology and of potential importance for the industry application of gas hydrate.