Abstract
Thermal compositional simulation can be challenging when narrow-boiling behavior is involved. The term “narrow-boiling” is used in the literature to refer to enthalpy that is sensitive to temperature. This paper presents an analysis of narrow-boiling behavior on the basis of multiphase isenthalpic-flash equations, where energy and phase behavior equations are coupled through the temperature dependency of K values. The Peng-Robinson equation of state is the thermodynamic model used in the analysis.
The general condition for narrow-boiling behavior is that the interplay between energy balance and phase behavior is significant. This is realized in engineering computations, such as flash calculations and reservoir simulation, as the sensitivity of K values to temperature. Two subsets of the condition are derived by analyzing the convex function whose gradient vectors consist of the Rachford-Rice equations; (i) the overall composition is near an edge of composition space, and (ii) the solution conditions (temperature, pressure, and overall composition) are near a critical point, including a critical endpoint. A special case of the first specific condition is the fluids with one degree of freedom, for which enthalpy is discontinuous in temperature.
Case studies are given to confirm the narrow-boiling conditions for water-containing hydrocarbon mixtures. Narrow-boiling behavior tends to occur in thermal compositional simulation likely because water is by far the most dominant component in the fluid systems formed in the simulation. K values can be sensitive to temperature for those fluids with skewed concentration distributions. Decoupling of temperature from the other variables is confirmed to be robust in isenthalpic flash for narrow-boiling fluids.