Example 2.9: Risk of condensation of water in a gas stream.

A light hydrocarbon flow is contaminated with water. The mixture is available at 200 kPa but due to severe weather conditions there is a risk of low temperature. Is there a real risk of condensation of water in the line?

Analysis:

Ambient temperature is in all possible cases greater than the critical temperature of methane. Ethane and propane could condense but the vapour pressure, even at 0 °C is greater than 200 kPa, so no hydrocarbons are expected to reach the organic liquid phase.

The pressure is given and a condensation temperature must be found. It is a dew point calculation.

Components are light hydrocarbons and water. Water is known not to mix with hydrocarbons in the liquid phase.

Phases are vapour and liquid. The liquid phase will contain only water.

Solution:

If water condenses the aqueous phase can be considered as pure, so with a composition equal to unity. Pressure is low so the Raoult approximation is valid. Hence, the fugacity of the liquid, incipient phase may be approximated with the vapour pressure of pure water. The fugacity of water in the bulk (vapour) phase is equal to its partial pressure. Hence, if the partial pressure of water reaches the vapour pressure at a given temperature, water will condense:

The form of the DIPPR equation for vapour pressure cannot be solved analytically in T. A trial and error procedure has to be implemented.

The maximum water content before liquid drops out is then found as .

Table 1 shows some results. If the temperature is below 0 °C then the sublimation pressure of water should be used and ice would appear.

Watch for the risk of hydrate formation if the pressure had been higher, for example in the case of flow assurance applications.