Gas hydrates are solid materials crystalline formed by the inclusion of certain molecules into the voids created by a three dimensional network of water molecules. The water molecules are linked together with hydrogen bonds. There is a great interest in the academia and in the industry to know the physical properties and the conditions under which these compounds are formed. The present thesis deals with the determination of the equilibrium conditions in multicomponent systems capable of forming gas hydrates. Particularly, it is concerned with the experimental and and theoretical determination of the inhibiting effect of electrolytes on the vapor-aqueous liquid-hydrate equilibrium conditions and on the calculation of the inhibiting effect of methanol.

A new experimental apparatus was designed and built. Its main component is a variable volume high pressure cell which is immersed in a temperature controlled bath. The cell is equipped with two marine-type windows made of plexiglas. The windows allow visual observation of the cell contents. The apparatus is shown to provide measurements which are reproducible and consistent with the measurements in other laboratories. for Experimental equilibrium data ethane hydrate formation in the presence of single and mixed electrolyte solutions of NaCl, KCl, CaCl and KBr were obtained using this apparatus. Fifty experiments were performed at temperatures between 265.36 and 282.98 K. First, data on hydrate formation in pure water and in single electrolyte solutions were obtained. Second, data on hydrate formation in four solutions of binary salt mixtures and a solution of the ternary salt mixture from NaCl, KCl and CaCl₂ were collected. Finally, a four-component aqueous solution was prepared and hydrate formation experiments were conducted.

A predictive method for calculating the incipient equilibrium formation conditions for hydrocarbon gas hydrates in the presence of single or mixed electrolytes was developed. The method utilizes the statistical thermodynamics model of van der Waals and Platteeuw describe the solid hydrate phase, the electrolyte activity coefficient model of Pitzer or that of Meissner to describe the aqueous electrolyte solution and the Trebble-Bishnoi equation of state for the vapor phase. The method is able to predict very accurately all the available experimental data in the literature as well as all the data obtained in this work.

A method was also developed for the calculation of the inhibiting effect of methanol on the equilibrium hydrate formation conditions. The method is distinguished from another method available in the literature because it uses only an equation of state to model all the fluid phases instead of three different models. The predictions were found to agree well with the experimental data available in the literature. The method is also used to calculate the amount of methanol required to inhibit hydrate formation in a natural gas stream and to calculate the amounts of methanol distributed in the vapor and liquid phases.

In the above methods the accuracy of the predictions relies on the ability of the thermodynamic models to describe the phases. The use of adjustable parameters, called binary interaction parameters, in equations of state improves dramatically their ability to represent the phase behavior of binary systems. In this work, a computationally efficient method for the estimation of these parameters is presented. It uses binary vapor-liquid equilibrium data and consists of a least squares estimation procedure to find the best combination of interaction parameters and a maximum likelihood estimation procedure to determine the statistically best parameter values. When three phase equilibrium data are available, they can easily be included in the parameter estimation database. The additional computational requirements are negligible. In addition, a constrained least squares interaction parameter estimation method was proposed. It enables the calculation of such parameters which when used in the thermodynamic model (equation of state) prevent the prediction of erroneous liquid phase separation. All the above estimation methods were illustrated with examples using the Trebble-Bishnoi equation of state.

Finally, the phenomenon of the supersaturation of the aqueous liquid phase with methane prior to methane hydrate nucleation was examined. An attempt was made to relate the limits of supersaturation with the liquid phase stability limits. In addition, tangent plane analysis for the identification of the phase behavior was performed for this system.