Hydrates In the PropaneCarbon Dioxide- Water System
Authors: Robinson D.B. , Metha B.R. 
Published: March 2010

The conditions for initial hydrate formation in systems containing carbon dioxide, propane and water were obtained over a wide concentration range for the hydrate-water-rich liquid-gas phase region. The locus of the four-phase equilibrium consisting of hydrate, water-rich liquid, gas and propane - carbon dioxide - rich liquid was determined. The maximum temperature at which hydrates would form in the three-component system was found to be 57.7 °F, compared to 50_3 DF for pure carbon dioxide and 42.3 °F for pure propane. An analysis of the data in terms of the solid-vapour K-factor concept indicated that the If-factors reported for propa.ne do not apply in systems containing carbon dioxide.

INTRODUCTION

A GAS HYDRATE is an inclusion compound in which water and at least one other component are associated through the enclosure of the hydrate-forming molecules in a crystalline lattice-like structure formed by the water molecules. The process of hydrate formation may be represented by:

(Equation Available In Full Paper)

where a hydrate-forming gas, R, combines with n molecules of water to form the solid crystalline hydrate. The forces stabilizing the structure are physical in nature, and the value of n may theoretically vary from 5.75 to over 17.0, depending on the hydrate-forming molecule and the structure of hydrate formed (1).

Many of the organic and inorganic gases form stable hydrates. These include, for example, carbon dioxide, hydrogen sulphide, methane, ethane, propane and nitrogen, all of which are commonly found in naturally occurring gas reservoirs. For this reason, information on the hydrate-forming conditions of these gases and their mixtures is of importance to the natural gas handling and processing industries. Although extensive experimental work has been reported in the literature on hydrate formation for the pure components, far less information is available on mixtures. Some of the systems for which information is available include methane – hydrogen sulphide(2), methane – carbon dioxide(3). methane-nitrogen(4), methane-propane(5) and propane hydrogen sulphide(6). Initial hydrate formation in several natural gas mixtures is reported by Deaton and Frost(7,8).

As an extension of the work reported above, it was decided to study initial hydrate formation in a system containing carbon dioxide and propane. Both of these components are commonly found in natural gas; furthermore, the system is of interest from a phase behaviour point of view, because pure carbon dioxide and pure propane form hydrates in different crystal structures.

Hydrate Structure And Phase Behaviour

The crystal structures in which hydrates are formed have been classified as Structure I and Structure II by Claussen(9) and von Stackelberg(10,11) and the theory pertaining to this is extensively reviewed by van der Waals and Platteeuw12. In this classification, carbon dioxide, a smaller molecule, forms in Structure I and propane, a somewhat larger molecule, forms in Structure II. The ternary system containing water, propane and carbon-dioxide exhibits two four-phase equilibrium consisting in one case of hydrate II (Hir), water-rich liquid (L1), gas (G) and propane – carbon dioxide – rich liquid (L2) and, in the other case, of hydrate I (Hr), water-rich liquid (L1), gas (G) and propane - carbon dioxide - rich liquid (L2).