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High Pressure Phase Behavior of Long Chain Alkanes: Solid-Solid and Solid-Liquid Transitions

Theo W. de Loos1, Jose J.B. Machado1, Christian Ihmels2, Kai Fischer2, and Juergen Gmehling3. (1) Process and Energy, Delft University of Technology, Leeghwaterstraat 44, Delft 2628 CA, Netherlands, (2) Laboratory of Thermophysical Properties LTP GmbH, (3) Industrial Chemistry, University of Oldenburg

An example of precipitation of undesired solid phases is the production of petroleum fluids from hyperbaric reservoirs which are found at increasing depths. This higher depths lead to a very asymmetric composition of the reservoir fluids, with a mole fraction of methane up to 0.6 and presence of heavy alkanes with carbon number up to 60. It is very common that a solid phase precipitates because of a change in the fluid pressure and temperature. A well known phenomenon is wax precipitation, the precipitation of solid phases made up of long chain heavy alkanes.

To get a better understanding of the phase behaviour of these hyberbaric reservoirs we are studying the phase behaviour of binary and ternary model reservoir fluids composed of methane and long-chain n-alkanes.

In this contribution, the thermal behavior, upon heating, was studied for 4 pure alkanes: eicosane, tetracosane, triacontane and tetracontane at pressures from 10 MPa to 150 MPa. The experiments were performed using the transitiometer developed and described by Randzio et al. [1].The calorimetric signal, pressure and temperature was measured using a very slow heating rate to guarantee thermal equilibrium.

It was found that from the compounds studied, eicosane was the only one that did not present a solid-solid transition; the other compounds show a solid-solid transition a few Kelvin below the solid-liquid transition temperature. This solid-solid transition disappears when the pressure is increased in a triple point of type: solid-solid-liquid.

1. S.L. Randzio, J. Therm. Anal. 46 (1996) 335-336.