Hydrate Inhibition via Alcohols and Glycols

Makogon (1981, p. 134) and Berecz and Balla-Achs (1983, p. 102) indicated that methanol can increase the temperature of hydrate formation at concentrations less than 5 mass% (presumably due to the clustering effect), but higher concentrations inhibit formation. Nakayama and Hashimoto (1980) also suggested that several of the alcohols could form hydrates yet further study by Nakayama et al. (1997) caused the opposite opinion. Further measurements by Svartas (1988) also indicated that small methanol amounts do not increase hydrate thermodynamic stability. 

Of alcohols, methanol has been the most popular inhibitor, due to its cost and its effectiveness. Katz et al. (1959, p. 218) indicated that the inhibition ability of alcohols increases with volatility, that is, methanol ethanol isopropanol. Typically methanol is vaporized into the gas stream of a transmission line, then dissolves in any free water accumulation(s) where hydrate formation is prevented. Makogon (1981, p. 133) noted that in 1972 the Soviet gas industry used 0.3 kg of methanol for every 1000 m3 of gas extracted. Stange et al. (1989) indicated that North Sea methanol usage may surpass the ratio given by Makogon by an order 

In a comprehensive set of experimental studies, Ng and Robinson (1983) determined that methanol inhibited hydrate formation more than an equivalent mass fraction of glycol in the aqueous liquid. The preference for methanol versus glycol may also be determined by economic considerations (Nelson, 1973). However, in many North Sea applications ethylene glycol is the preferred inhibition method. 

Techniques for hydrate inhibition deal with the methanol concentration in the aqueous liquid in equilibrium with hydrate at a given temperature and pressure. The user also must determine the amount of methanol to be injected in the vapor. This problem was addressed first by Jacoby (1953) and then by Nielsen and Bucklin (1983), who presented a revised methanol injection calculation. The most recent data are by Ng and Chen (1995) for distribution of methanol in three phases (1) the vapor phase, (2) the aqueous phase, and (3) the liquid hydrocarbon phase. 

To approximate the hydrate depression temperature for several inhibitors in the aqueous liquid, the natural gas industry uses the original Hammerschmidt (1939) expression to this day as a check  

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