  Gas hydrates grown at gas-ice interfaces are examined by electron microscopy
and found to have a submicron porous texture. Permeability of the intervening
hydrate layers provides the connection between the two counterparts (gas and
water molecules) of the clathration reaction and makes further hydrate
formation possible. The study is focused on phenomenological description of
principal stages and rate-limiting processes that control the kinetics of the
porous gas hydrate crystal growth from ice powders. Although the detailed
physical mechanisms involved in the porous hydrate formation still are not
fully understood, the initial stage of hydrate film spreading over the ice
surface should be distinguished from the subsequent stage which is presumably
limited by the clathration reaction at the ice-hydrate interface and develops
after the ice grain coating is finished. The model reveals a time dependence of
the reaction degree essentially different from that when the rate-limiting step
of the hydrate formation at the second stage is the gas and water transport
(diffusion) through the hydrate shells surrounding the shrinking ice cores. The
theory is aimed at the interpretation of experimental data on the hydrate
growth kinetics.
