• The inhibitory performance of alanine as a hydrate inhibitor was investigated. At a concentration of 0.6 wt%, alanine achieved the longest induction time and the lowest total gas consumption; specifically, the induction time was 107% longer than that in the pure water system. • The effects of sodium chloride (NaCl) on the formation of CO 2 hydrates were investigated. The results showed that with the increase in NaCl concentration, the amount of CO 2 hydrate formation decreased significantly; furthermore, when the NaCl concentration increased to 8 wt%, the induction time was prolonged remarkably and the amount of hydrate formation was reduced drastically. • When sodium chloride (NaCl) is combined with alanine (0.6 wt%) as a hydrate inhibitor, the inhibitory effect is significantly enhanced at medium to high salt concentrations (4 and 6 wt%); at high salt concentrations (8 wt%), the formation of hydrates is almost completely inhibited. • The morphological characteristics of hydrates in different systems were analyzed. In both the pure water system and the alanine-only system, the hydrates appeared milky white; while in the mixed system of 0.6 wt% alanine and 6 wt% NaCl, only a small amount of ice-like hydrates were present at the gas-liquid interface of the reactor. • The hydrate formation mechanisms of the single alanine system, single sodium chloride system, and the alanine-sodium chloride composite system were analyzed separately. (1). For the single alanine system, existing literature studies have shown that the amino and carboxyl groups of alanine can form a hydrogen bond network with water molecules, disrupting the ordered arrangement of water molecules, thereby increasing the nucleation energy barrier and delaying hydrate formation. In addition, alanine with strong hydrophobicity and a short alkyl chain exhibits a significant inhibitory effect on hydrate formation; for low concentrations of alanine, the possible mechanism is that alanine molecules aggregate, thus impairing their effective activity at the gas-liquid interface. (2). For the single sodium chloride system, Na⁺ and Cl⁻ can enter the hydrate structure as lattice sites, causing lattice distortion of hydrate crystals and affecting the hydrate formation rate. At high concentrations, large NaCl clusters are formed and expelled outside the interface, making it difficult for hydrates to grow. (3). In the composite system, the electrolyte environment provided by sodium chloride promotes a more uniform dispersion of L-alanine molecules and enhances their interfacial activity, thus further prolonging the induction period and reducing the gas consumption rate. Additionally, sodium chloride can enhance the thermodynamic inhibition effect of alanine. During CO 2 pipeline transportation, the low-temperature and high-pressure environment readily promotes the formation of CO 2 hydrates, which can lead to pipeline blockage, pressure fluctuations, and even catastrophic failures. In this study, L-alanine (Ala), sodium chloride (NaCl), and their combined systems were investigated to systematically evaluate their effects on the kinetics of CO 2 hydrate formation. Experiments were conducted in a high-pressure visual reactor under the initial conditions of 3.5 MPa, 12°C, and a stirring speed of 270 r/min.The results showed that Ala could effectively prolong the induction time of hydrate formation and inhibit hydrate growth, with an optimal inhibition concentration of 0.6 wt%, extending the induction time to 45.2 min, an increase of 107% compared with the pure water system. NaCl not only extended the induction time but also significantly inhibited hydrate growth and reduced the hydrate formation amount. Based on the optimal concentration of Ala, NaCl was introduced as a co-inhibitor, and it was found that when the NaCl concentration exceeded 6 wt%, the induction time, growth rate and formation amount of hydrates were all significantly inhibited. Particularly at 8 wt% NaCl, no hydrate formation was observed within 2500 min, demonstrating a strong synergistic inhibition effect. Further analysis revealed that Ala disrupts the ordered hydrogen-bond structure of water through hydrogen-bond interactions, while NaCl reduces water activity and induces lattice distortion in hydrate crystals, thereby affecting both the induction time and the amount of hydrate formed. Through their combined action, nucleation and growth of CO 2 hydrates were effectively suppressed. This study provides both theoretical and experimental support for developing green, efficient CO 2 hydrate inhibitors suitable for pipeline transportation applications.
Chai et al. (Sun,) studied this question.