Phase diagrams for hydrates beyond incipient condition — Complex behavior in methane/propane and carbon dioxide/iso-butane hydrates

Abstract

We have recently developed an algorithm for multiphase equilibrium calculations and stability analysis involving hydrates. This algorithm allows phase equilibrium calculations in scenarios presenting different number of phases that may also be in distinct physical states.Here, we present P×T phase diagrams calculated using our algorithm that involve mixed hydrates of natural gas beyond incipient condition and show complex behavior. Our phase diagrams show a global overview of the behavior of phase equilibrium involving hydrates in a mixture of water, methane and propane and in a mixture of water, carbon dioxide and iso-butane. We accomplish that using a global phase diagram in a wide range of pressure and temperature to show diverse phase equilibrium regions.We present an approach to tune the Peng–Robinson equation of state to improve the calculations of the chemical potential of water in liquid state at high pressure and therefore, improve the prediction of the phase equilibrium between this phase and ice or hydrates, up to pressure conditions as high as 100 MPa.Our calculations predict 3-phase equilibrium regions and 4-phase equilibrium univariant lines in binary hydrates of methane and propane and in binary hydrates of carbon dioxide and iso-butane. We use local phase diagrams highlighting details in regions with complex behavior of phase equilibrium to show how the phase boundary isopleths and univariant lines relate in a phase diagram for water, methane and propane.Our calculations for a mixture of water, carbon dioxide and iso-butane predicts structural transitions occurring below the dew point line of the guest components, in a mixture with excess of guest components, and 5- phase equilibrium invariant point. We show equilibrium lines that present retrograde behavior in the phase diagram for this mixture.