Kinetic and Morphological Investigations on the Effect of Hydrophobic and Hydrophilic Amino Acids during Hydrate Based Gas Separation of CH₄─CO₂ (Biogas)

Biogas has been playing a pivotal role in energy transition, and its production has been increasing globally. Upgradation of biogas is essential for enriching the calorific value of biogas. For the purpose of separating out CO2 from biogas and producing purified CH4, hydrate based technology has been proposed. In the present study, we examine for the first time the kinetics of hydrate formation employing a 50–50 mol % mixture of CH4–CO2 in the presence of amino acid additives at selected concentrations and operating conditions (T and P). Kinetic parameters of gas separation like recovery of CO2, separation factor, gas uptake, t90, and residual and hydrate gas composition have been studied. Concentrations of amino acids and driving force were varied to study their effect on the kinetics of hydrate formation, enabling the identification of optimal process conditions to achieve a higher separation factor alongside improved gas uptake. Three hydrophobic (l-methionine, L-tryptophan, and l-phenylalanine) and two hydrophilic amino acids (arginine and histidine) at concentrations of 0.1 and 0.5 wt % were chosen to see their effect on separation of the biogas mixture along with kinetics under comparable temperature and pressure conditions. It was seen that hydrophobic amino acids act as efficient additives in enhancing the hydrate formation rate yet result in poor separation efficiency. On the other hand, hydrophilic amino acids showcase a higher separation efficiency but with reduced gas uptake/gas recovery at 274.2 K and 6.9 MPa/4.9 MPa. The highest separation factor of 8.24 ± 0.37 was achieved using 0.1 wt % l-arginine at 3 MPa driving force. Further, morphological observations during hydrate formation using different amino acids at performed experimental conditions have been presented.