Effect of Carbon Chain Length of Organic Salts on the Thermodynamic Stability of Methane Hydrate

Abstract

This study presents the phase equilibrium conditions for methane hydrate with one of the following organic ammonium salts differing in carbon chain length: tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB), tetrapropylammonium bromide (TPrAB), tetrabutylammonium bromide (TBAB), and tetrapentylammonium bromide (TPeAB). The hydrate phase equilibrium measurements were conducted for a temperature range of 278.94–291.85 K and pressure range of 4.79–14.32 MPa using the step-heating pressure search method. The addition of TBAB or TPeAB shifts the phase equilibria of the semiclathrate hydrates (SCHs) of CH4 to a lower pressure and higher temperature zone. At a given temperature, increasing the mole fraction of TBAB and TPeAB from 0.294 mol % to 0.620 mol % made the shift in phase equilibrium conditions greater. At a given dosage, TBAB consistently outperformed TPeAB in thermodynamically promoting methane hydrate formation. TMAB, TEAB, or TPrAB slightly shifts the phase equilibrium conditions to a higher pressure and lower temperature region. We analyzed the hydrate phase equilibrium data for TMAB, TEAB, and TPrAB using the colligative property equation and compared them with the phase equilibrium data of a CH4 and salt water system. The results suggest that these three organic salts have a small hydrate inhibiting effect that is comparable to NaCl. Promotion of the formation of CH4 hydrate by TBAB and TPeAB indicates that these additives provide a means to store CH4 at moderate pressure conditions, which could lower the cost of pressure reduction in hydrate formation. In contrast, TMAB, TEAB, and TPrAB could be used for prevention of formation of hydrates in systems where the use of NaCl is unsuitable.

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

“…Phase equilibrium data on methane hydrate + (TEAB, TMAB, and TBAB) aqueous solutions with varying concentrations have also been used and compared from various literature. 5,10,11,20 The results on 0.05 mf of TMAB and TEAB obtained in this work are in good agreement with those obtained by Su and co-workers. 20 It has been observed from Figure 3 that TBAB shows significant hydrate promotion, while TMAB and TEAB show hydrate inhibition.…”

“…Very few studies have been reported for methane hydrate formation in the presence of varying alkyl chain length of QASs in an aqueous solution. Recently, Su and co-workers 20 have investigated the phase equilibrium of methane hydrate + QASs aqueous solutions, such as TMAB, TEAB, TPrAB, TPeAB, and TBAB. TMAB, TEAB, and TPrAB were investigated at a single concentration (0.62 mol %), whereas TPeAB and TBAB were studied at two different concentrations (0.294 mol % and 0.62 mol %).…”

Thermodynamic modeling of gas solubility in aqueous solutions of quaternary ammonium salts with the e-CPA equation of state

“…As for the effects on hydrate formation, gas solubility is just one factor. Su et al [87] and Gupta et al [88] observed that TBAB is a hydrate promoter, but TPAB, TEAB and TMAB are hydrate inhibitors.…”

Review on Application of Quaternary Ammonium Salts for Gas Hydrate Inhibition

“…In evaluating the performance of QAS for gas hydrate inhibition, several methods has been applied, such as the T-cycle method and using highpressure micro differential scanning calorimetry equipment. The Isochoric constant cooling method or T-cycle method is one of the most used method, carried out by [37,[56][57][58][59][60][61][62] with different type of gases to study on the kinetic inhibition of the selected compounds. In this method, 100 mL of liquid sample is poured into the cell which is then vacuumed thoroughly to remove excess air that may still exist in the cell [20,63].…”