Desalination Via Formation Of Binary Clathrate Hydrates

Abstract

Desalination through formation of single-component hydrates, actively pursued in the 1970s, suffers from fact that these hydrates usually require low temperatures and/or high pressures to form. This dissertation focuses on binary clathrate hydrates, where the larger hydrate former occupies the larger cavities of a sII-type hydrate and the smaller helper gas occupies the smaller cavities. Binary hydrates, which can be formed at higher temperatures and/or lower pressures than single-component hydrates, could potentially lower the cost of hydrate-based desalination.

In this dissertation, the thermodynamics and kinetics of formation of cyclopentane-methane and cyclopentane-carbon dioxide hydrates in fresh water and brine solutions are studied experimentally. Hydrate formation experiments were conducted in a stirred tank in the 280-295K and 1-3 MPa range and at different stirring rates. From the temporal evolution of pressure and temperature, the hydrate formation rate and the fractional occupancy of the small cavity by the helper gas were extracted. In freshwater, the hydrate growth rate mainly depends on the degree of subcooling and the extent of hydrate formation, with the mass transfer of the helper gas being an important rate-controlling factor. Salt is found to affect the binary hydrate formation process in two ways. First, it shifts the hydrate equilibrium curves so it takes a higher pressure (at the same temperature) to achieve the same degree of subcooling. Second, the concentration polarization associated with rejection of salt by the growing hydrates particles imposes a moderate mass transfer resistance on the hydrate formation process. Nevertheless, the binary hydrate growth rate in brine solutions is found to be comparable to that in fresh water under the same subcooling level.

Poor mixing was found to be an issue during the formation of cyclopentane-carbon dioxide hydrates, but not in the case of cyclopentane-methane hydrates. Nevertheless, the hydrate formation rates for both systems were found to be comparable.

A new displacement washing procedure to separate the hydrate crystals from brine has been developed and experimentally tested, demonstrating good hydrate recovery ratio and washing efficiency.