Question

Suppose we have a closed container not isolated from the external environment so that heat exchanges can occur on the lateral wall (mainly for convection). The water inside is pressurized by a gas (nitrogen for example) starting from a pressure that is not exceeding 20 bar.

As water is drained from the tank I'm expecting that pressure will decrease as a result of the expansion and so will temperature that will lead to the onset of heat exchanges between liquid and gas and internal lateral walls. At this point a question comes: If a thermal control is inserted in the tank so that liquid temperature stays always in the range of 10 - 30 degree Celsius, boiling will occur only if vapor pressure of liquid will equate the pressure exerted by nitrogen on the liquid free surface. Can evaporation take place before reaching this condition? In this case, it is possible to consider that phase equilibrium is present?

I'm saying this since I can expect that some water molecules will have enough energy to escape liquid but will eventually come back due to volume constraint (even in the case of small quantities of vapor).

The same can be said also when boiling happens in such a way that its onset is always accompanied by condensation?

Answer 1

No, evaporation can occur before reaching the condition where the vapor pressure of the liquid equals the pressure exerted by nitrogen. Phase equilibrium may not be present in this case.

Step-by-step explanation:

In a closed container with heat exchange, as water is drained and pressure decreases, the temperature also drops. According to the phase diagram for water, the saturation temperature at 20 bar is higher than the desired temperature range of 10 - 30°C. This means that as the pressure decreases, the water temperature will drop below the desired range before reaching the equilibrium condition for boiling. Therefore, evaporation can occur before reaching the boiling condition.

The presence of a thermal control to maintain the liquid temperature in the desired range ensures that the water remains within the specified temperature limits even as it drains. However, evaporation may still occur due to the inherent energy distribution of water molecules.

Some molecules may have enough kinetic energy to escape the liquid phase temporarily, forming vapor. Yet, due to volume constraints and the thermal control mechanism, these molecules are likely to return to the liquid phase, preventing the establishment of a true phase equilibrium.

In the case of boiling, the onset is typically accompanied by condensation. Boiling is a phase change from liquid to vapor, and it involves the absorption of latent heat. Condensation, on the other hand, is the transition from vapor to liquid, releasing latent heat. Both processes occur simultaneously during boiling until the entire liquid is vaporized. The interplay between evaporation, condensation, and the thermal control system influences the overall behavior of the system.