Produced waters, due to the presence of salts, have a hydrate inhibiting effect. However, the data on hydrate formation in the presence of electrolytes is scarce. This paper presents experimental and modelling results on the effect of salts on hydrate equilibria for ethane and propane simple gas hydrates. This is a part of an ongoing research project on the hydrate inhibition effect of electrolytes in relation to subsea transmission lines where the proportion of water could be significant.
The conventional graphical method, backed up with visual observation has been used in hydrate dissociation point determinations. For ethane hydrates, dissociation conditions have been determined in the presence of 20 Wt% NaCl solution, while for propane, aqueous electrolyte solutions of NaCl, KC1, and CaCl2, with concentrations ranging from 3 to 20 Wt% have been used.
Both Valderrama (VPT) EoS with Non Density Dependent (NDD) mixing rules and Peng-Robinson (PR) EoS have been used for the fluid phases. The effect of salts in the water phase has been modelled by adding an electrostatic contribution to each EoS, with parameters from vapour pressure depression in electrolyte solutions. For the hydrate phases the model is coupled with the ideal solid solution theory.
The predictions of the model are compared with experimental and literature data and good agreement is demonstrated.