Phase Equilibrium of Methane Hydrate in the Presence of Aqueous Solutions of Quaternary Ammonium Salts

Families of various quaternary ammonium salts (QAS) have been of great interest to gas hydrate based investigations. In this work, an attempt has been made to understand the effect of QAS of the bromide family with increasing alkyl chain length, such as tetra-methyl, tetra-ethyl, and tetra-butyl ammonium bromide (TMAB, TEAB, and TBAB) at two different concentrations (0.05 and 0.1 mass fraction) in an aqueous solution on the hydrate-liquid-vapor (H-L-V)phase equilibrium of the methane hydrate system. Various experiments were performed to capture phase equilibrium data in the equilibrium pressure range of 7.6-4.2 MPa and temperature range of 282.4-276.8 K. It has been observed that the addition of TMAB and TEAB shifts the phase equilibrium curve of methane hydrate to higher pressure and lower temperature conditions. TMAB and TEAB have shown thermodynamic inhibition unlike TBAB which has shown a promotion effect. The Clausius-Clapeyron equation is used to calculate the enthalpy of dissociation of methane hydrate in various QAS aqueous solutions to examine the effect of QAS on methane hydrate structural information. The electrical conductivity measurements were also made to correlate the hydrate inhibition effectiveness of QAS on methane hydrate system. In addition, a phase equilibrium model has been extended to predict the phase behavior of methane hydrate + (TMAB, TEAB, or TBAB) aqueous solutions for a total 91 experimental phase equilibrium data points obtained from this work and the literature. The absolute average relative deviation in equilibrium pressure (AARD/P (%)) observed from the proposed model with the experimental equilibrium pressure data produced in this work and from several sources in the literature have been observed to lie within 3.2%, indicating the robustness of the model.