Unusual promotion/inhibition effect of polyethylenimine on CO2/CH4 hydrate formation: A potential green chemical additive for CH4-CO2 replacement process

The unusual behavior of a new class of compounds that selectively function as either promoters or inhibitors of gas hydrate formation depending on the process conditions has made their exploration increasingly appealing. In this study, polyethylenimine (PEI) exhibits desirable functionality that may be a candidate for environmentally benign compounds. The present work investigated PEI's thermodynamic and kinetic influence on the formation of CH4 and CO2 hydrates for two PEI concentrations (0.5 and 5.0 mass%) and two PEI molecular weights (25,000 and 750,000 g/mol). The isochoric method was applied to measure the hydrate-liquid water–vapor-three-phase-coexisting (H-LW-V-3PC) phase boundary. PEI is a thermodynamic inhibitor regardless of CH4 or CO2 as a hydrate former, and 0.5 mass% PEI system exhibits the highest average suppression temperatures of 0.25 and 0.62 K, respectively. Kinetic findings, such as induction time, initial hydrate formation rate, and nucleation rate, on the other hand, revealed that PEI could act differently in response to specific hydrate formers. PEI kinetically inhibits CH4 hydrates formation but promotes the formation of CO2 hydrates. This promotion effect is attributed to PEI's high CO2 absorption ability to enhance the solubility of CO2. This unique behavior of PEI presents appealing advantages in the natural gas hydrate exploitation field due to their potential for greater operational flexibility and minimal environmental impact.