Crystallisation of heavy hydrocarbons from three synthetic condensate gases at high pressure

In order to get a better understanding of the phase diagrams of natural condensate gases, three synthetic mixtures of known composition were investigated. The major components were methane (approximately 79 mol%), and pentane (approximately 20 mol%). A heavy hydrocarbon fraction (0.5 mol%) was composed either of n-tetracosane, naphthalene or both components. The detailed phase diagram of each mixture was determined in a fully visible sapphire cell, from 10 to 40 MPa and from 277 to 420 K. This study confirms the shape of the (gas)/(gas+solid) and (gas+liquid)/(gas+liquid+solid) phase boundaries. These phase boundaries converge at a single common point with the (gas)/(gas+liquid) and (gas+solid)/(gas+liquid+solid) phase boundaries. The investigated fluids also illustrate the various possible locations of the (gas=liquid) critical point. The phase diagrams could be modelled qualitatively with the Peng–Robinson equation of state, assuming that the heavy hydrocarbons crystallised as pure phases. The influence of pressure and of solid/solid phase transitions is discussed.