Collective Impact of Amino Acids on Clathrate Hydrates of the CO₂ + CH₄ Gas Mixture

Abstract

The amino acids are imposing additives in the gas-hydrate-based technology, and they play a dual role as inhibitors or promoters, primarily depending upon the concentration. Additionally, the nature of the side chain and its length are critical in the process. l-Phenylalanine (l-Phe) and l-threonine (l-Thr) are classified as hydrophobic and hydrophilic, respectively, and are more potent inhibitors for CO2 hydrates, while they promote CH4 hydrates. On the other hand, l-methionine (l-Met) and l-valine (l-Val) are good promotors for CO2 and CH4 hydrates. The present investigation is on the hydrate formation with mixed gas (CO2 + CH4) using the aqueous mixture of 80% l-Phe (or l-Thr) + 20% l-Met (or l-Val). The gas uptake in (l-Phe + l-Met) remained synergistically high, irrespective of the feed gas composition, while it suddenly became low in l-Phe + l-Val if the feed gas became rich in CO2. The CH4 hydrate formation in an aqueous mixture with (l-Thr + l-Met) is significantly inhibited; however, the gas uptake progressively increased with an increasing CO2 fraction in the feed. The gas uptake in (l-Thr + l-Val) is opposite the (l-Phe + l-Val) system; i.e., it remained low below the 50% CO2 fraction in the feed and remained synergistically high after that. The micro-Raman studies confirm encasing CH4 and CO2 molecules from the feed gas mixture.