(74b) Gas Hydrate Phase Behavior in Presence of Organics and Electrolytes

In this contribution, the effects of electrolytes on the phase behaviour of carbon dioxide hydrate forming systems are presented. The phase behaviour of the simple carbon dioxide hydrate system, the mixed carbon dioxide - tetrahydrofuran (THF) hydrate system, and, finally, the effect on the phase behaviour of different overall concentrations of sodium chloride will be elucidated. In addition, the competing effects between THF and an electrolyte of the metal halide series and their impacts on the phase behaviour of the hydrate forming systems will be discussed. The strength of hydrate inhibition by metal halides is compared and, a quantitative analyses between the effect of anions and cations is made in order to gain some understanding on the mechanism of electrolyte inhibition on hydrate formation.

It is well established that electrolytes inhibit clathrate hydrate formation. The inhibition of hydrate formation by electrolytes is due to the lowering of the activity of water in the coexisting liquid phase, causing hydrates to form at lower temperatures and higher pressures compared to their formation in pure water [1]. Many phase equilibrium data especially on H-L_W-V equilibrium line have been reported for carbon dioxide in the presence of electrolytes. A compilation of these data can be found in Sloan [2].  In contrast, cyclic organics such as tetrahydrofuran reduce the pressure requirement for hydrate formation at a specified temperature when presents in small quantity in aqueous solutions up to about 5 – 7 mol%. The usage of such organics at low concentration may provide an alternative solution to overcome the inhibiting effects of electrolytes. Unfortunately, no available reported data for hydrate equilibria of mixed carbon dioxide and tetrahydrofuran hydrates in aqueous solution have been found in literature. Realizing the potential benefits of such data for the development of hydrate-based processes, an attempt has been made to measure the hydrate equilibrium data for such a system. For consistency, tetrahydrofuran is chosen as a representative of hydrate promoter in the present work. In this study, phase behaviours of mixed carbon dioxide and tetrahydrofuran hydrates in sodium chloride (NaCl) solutions are presented. Sodium chloride is chosen to represent electrolytes that are present in naturally occurring waters. The measurement of hydrate equilibrium conditions for mixed carbon dioxide and tetrahydrofuran hydrates are extended to another six electrolytes from the metal halides series. Besides NaCl, the electrolytes used in this work are calcium chloride (CaCl₂), magnesium chloride (MgCl₂), potassium bromide (KBr), sodium fluoride (NaF) and sodium bromide (NaBr) and potassium chloride (KCl). Based on measured data, a comparison is made to quantitatively evaluate the effect of different salts inhibition on hydrate formation.

In the quaternary system studied, the presence of electrolyte in the system enhanced the liquid-liquid phase split creating two distinct liquid phases of liquid water and liquid organics (tetrahydrofruan + carbon dioxide). This behaviour is attributed to the salting-out effect that taking place in the system due to the presence of ions. In general, it is found that the hydrate promoting effect of tetrahydrofuran suppresses the inhibiting effect of an electrolyte when both are present in a hydrate forming system. The strength of hydrate inhibition effect among the electrolytes was compared. The results shows the hydrate inhibiting effect of the metal halides is increasing in the order NaF < KBr < NaCl < NaBr < CaCl₂ < MgCl₂. Among the cations studied, the strength of hydrate inhibition increases in the following order: K^-< Na^- <Ca²⁺ < Mg²⁺. Meanwhile, the strength of hydrate inhibition among the halogen anion studied decreases in the following order: Br^- > Cl^- > F^-. Based on the results, it is suggested that the probability of formation and the strength of ionic-hydrogen bond between an ion and water molecule and the effects of this bond on the ambient water network are the major factors that contribute to hydrate inhibition by electrolytes.

 References

[1] Z. Duan, R. Sun, American Mineralogist, 91 (2006) 346-1354.

[2] E.D. Sloan Jr., C.A. Koh, “Clathrate Hydrates of Natural Gas”, 2^nd ed., CRC Press, Florida, 2008.