HYDRATE STRUCTURE STABILITY IN SIMPLE AND MIXED HYDRATES

In the course of work to develop a new and consistent model of hydrate s and hydrate inhibitors we have made some unexpected predictions abou t the structure of hydrates that appear to be supported by the current ly available experimental data. Based on the data and on the accepted theory of thermodynamic stability, we conjecture, that the hydrate for med by certain mixtures is structure II - in a certain composition ran ge - whereas the associated simple hydrates are structure I. This is t rue for a number of ternary systems such as methane-ethane-water. The structure of nitrogen hydrate has been the subject of some debate. We found that it is impossible to derive a consistent set of model parame ters for the nitrogen simple hydrate, assuming nitrogen forms structur e II at ambient temperatures. However, structure II will form from a m ixture of nitrogen with very small amounts of ethane or propane added. The phase transitions are a natural consequence of the physical facto rs involved: empty lattice stability; number of cages in each structur e; affinity of hydrate former for each cage. The conclusion about whic h is the stable hydrate structure can be drawn from a fairly simple an alysis of the equations and holds equally well for several models publ ished by other authors. Our discovery was, however, a consequence of t he application of rigorous and consistent multiphase-split algorithms. Experimental confirmation would give extra support to the current the ory and parameters. Precise measurement of the conditions at which the transitions occur would be a very useful contribution to the determin ation of the correct hydrate model parameters.