Modeling of thermodynamic conditions for the recovery of high-purity helium from gas mixtures by the hydrate formation method

Abstract

The modern technology for the recovery of helium from heliumcontaining natural gas is based on the lowtemperature separation (fractional distillation) method, in which helium is separated from other gases in the course of their liquefaction. This method requires considerable energy and resource inputs. An energysaving route to separate gas mixtures can be enrichment of natural gas with helium upon clathrate hydrate formation (1). Gas (clathrate) hydrates are inclusion compounds consisting of water molecules, which form the host crystal lattice, and guest molecules occupying lattice cavities (2). Current interest in gas hydrates stems from their possible use as fuel, as well as from their possible effect on global climate changes (3) and the use of gas hydrate technologies for natural gas storage and transportation (4). The helium content in natural gas is usually low as compared with the major component methane, although there are some natural gas deposits contain� ing up to 8 vol % helium. Methane accounts for more than 70-80 vol % of such gas deposits. Commercially interesting for helium recovery are natural gas deposits containing more than 0.3 vol % helium (5). The exist� ence of clathrate helium hydrates is still debatable since no reliable experimental data supporting or refuting this fact have been available so far. However, experimental evidence for the possibility of helium hydrate formation has been reported in (6). It has been theoretically predicted that structure II helium hydrates can form at high pressures (beginning with 800 atm) and low temperatures (250 K) (7). It has been shown that, at the same temperatures, mixed structure I and II methane-helium clathrate hydrates form at lower pressures than pure helium hydrates even at small concentrations of methane in the gas phase (8). The present work is aimed to find a correlation between the composition of mixed methane-helium hydrates of cubic structures I and II and the composi� tion of the gas phase being at equilibrium with the hydrate, as well as to determine conditions of forma� tion of mixed methane-helium hydrates at low helium concentrations in the gas phase for developing meth� ods of deep purification of helium from methane via hydrate formation.