Vapor-liquid-water equilibrium (VLWE)

In the Fluid Editor, in the System section, you can select Vapor/Liquid/Water (VLWE) (Beta Version) for the Phases option for compositional fluids. The vapor-liquid-water equilibrium (VLWE) includes a vapor phase, a liquid phase, and a water decant phase.

For most hydrocarbon-water mixtures found in refinery and gas plant operations, the free water phase that forms is nearly immiscible with the hydrocarbon liquid phase. For such systems, we can model the three-phase calculation by decanting the free water as a pure aqueous phase. This greatly simplifies the VLLE calculations.

For the VLWE calculations, we assume that the water vapor forms an ideal mixture with the hydrocarbon vapor phase. We predict the quantity of water in the vapor phase by using an equilibrium calculation, which uses the following expressions for the K-value of water.

where

KwI is the K-value of water between the vapor phase and the hydrocarbon liquid phase

KwII is the K-value of water between the vapor phase and the free water phase

yw is the water composition in the vapor phase

xwI is the water composition in the hydrocarbon liquid phase, which is a measure of the water solubility in the hydrocarbon liquid

xwII is the water composition in the free water phase

Pwsat is the water saturation pressure at the current temperature

P is the current pressure

We use the Industrial Formulation 1997 (IF97) steam tables from the International Association for the Properties of Water and Steam (IAPWS) to calculate KwI and KwII.

For KwII, we assume that the free water phase is pure water (xwII = 1.0). The K-value equation reduces to the following equation:

This K-value formulation allows the free water phase to form or disappear based on the system conditions. When the calculated water saturation pressure is less than the system pressure (Pwsat < P), the free water phase forms. When the calculated water saturation pressure is greater than the system pressure (Pwsat > P), the K-value is greater than 1.0 (KwII > 1.0). In this case, we must reduce the water composition in the free water phase to a value less than 1.0 (xwII < 1.0) so that the value of the water composition in the vapor (yw) does not exceed 1.0. When we reduce the xwII value, the free water phase effectively disappears.

When the free water phase is present, you can use the Separator model in the Process library to decant the water stream and separate the hydrocarbon liquid phase from the free water phase.

Limitations

• Components in the fluid cannot be miscible with water.

• You should not use VLWE with liquid activity methods.

• You should not use VLWE in high pressure systems.

• You should not use VLWE in cryogenic systems. The temperature must be above the freezing point of water at all times.

• In Dynamics mode, the vapor phase may not condense into a liquid phase as expected.

• You can use VLWE in the Column model. However, testing is still in progress and we currently do not officially support VLWE in the Column model.

• Currently, you can use only the API 68 Kerosene method to calculate the water solubility in the hydrocarbon liquid phase (xwI).