Water Decant Options

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 the three phase calculations may be modeled by decanting the free water as a pure aqueous phase. This greatly simplifies the VLLE calculations.

The free water decant option follows these steps:

1. The water vapor is assumed to form an ideal mixture with the hydrocarbon vapor phase.

2. The quantity of water in the vapor phase is predicted with an equilibrium calculation which uses the following expression for the K-value of water:

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where:

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P = system pressure

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See Water Partial Pressure Calculations and Water Solubility Methods for more information.

3. The vapor phase is checked for water saturation. If the partial pressure of the water in the vapor ( mole fraction times system pressure) exceeds the partial pressure for water at the system temperature, free water will form.

4. When free water will form, the vapor is saturated with water, i.e., the water content of the vapor is set such that the partial pressure of water in the vapor is equal to the saturation pressure for water at the system temperature

5. Water is dissolved in the hydrocarbon liquid up to its solubility limit.

6. The remaining liquid water is decanted as a pure water stream.

The free water decant option is limited to these K-value (VLE) methods: Soave-Redlich-Kwong, Peng-Robinson, Grayson-Streed options, Chao-Seader options, Braun-K10, and Benedict-Webb-Rubin-Starling. The water decant option is automatically activated when any of these methods is selected.

Additionally, the free water decant option can be used with the Regular Solution theory, which is generally considered to be a liquid activity method. However, water decant is possible because Regular Solution Theory includes simplifying assumptions from Scatchard and Hildebrand that consider liquid activity coefficients as functions only of pure component properties. The interfacial dependencies in other liquid activity methods generally prevent using the the water decant option.