ABSTRACT

The inhibiting effect of methanol (MeOH) on I, methane hydrate formation, given as a depression (ΔT) of the equilibrium, hydrate formation temperature has been examined for 0.57, 1.97, 5.44, and 20.01 % by weight (wt. %) MeOH in the original aqueous phase. For all concentrations of MeOH, a slow increase of ΔT with increasing pressure was observed. For pressures above approx. 100 bara, the measured ΔT's did not deviate much from values of ΔT calculated by the Hammerschmidt equation (Hammerschmidt 1934). A stabilization of the methane hydrate was not! observed for concentrations of MeOH below 5 wI. % contrary to reports by Makogon (1981) and Berecz and Balla-Achs (1983). The present study showed that, at equilibrium, MeOH in low concentrations inhibits methane hydrate formation for all pressures examined.

INTRODUCTION

Hydrate formation can cause severe problems during petroleum production and pipeline transport at temperature (- and pressure) conditions above the freezing point of water. Natural gas hydrates (clathrates) are ice-like crystals which form when unpolar, small sized gas molecules (e.g. hydrocarbons C1 to C4) stabilize an arranged structure of water molecules by entering certain cavities in the lattice. Two forms ofl hydrate, structures, structure I and II, a~e known to exist. Structure I is formed by inclusion of the smaller hydrocarbons like methane and ethane (and other small sized gas molecules like C02, H2S and N2), while structure II is normally formed by inc;lusion of the larger propane and butane molecules. When mixtures of structure 1- and structure p- forming hydrocarbon gases are involved, structure \I hydrate, being the more stable of the two, is preferentially formed. Structure I formers can enter the cavities of structure \I while propane and butane are too large to suit the cavities of structure I.

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