Adiabatic process: Can it cause phase change?

The answer is no.

The phase change does not happen arbitrarily. It's a state when the two phases go into thermodynamic equilibrium with minimum Gibbs free energy and maximum entropy.

At phase change of a single component system by Gibbs phase rule, F = C - P + 2, there is one component, C = 1, and there are two phases, P = 2, therefore, the degree of freedom, F = 1, meaning for a single component system a phase change is dictated by one intensive independent variable. It could either be pressure or temperature. When one of them is fixed the other dependent variable gets automatically fixed. For every saturation temperature on a phase (PT) diagram, there is a fixed saturation pressure and vice-versa, simply because one intensive independent variable controls a phase change.

How does phase change occur?

A phase change occurs when the substance in its current phase is saturated with energy. This means the existing phase has no more heat capacity to absorb any more heat and increase its sensible heat and temperature. At this point, the temperature rise stops despite supplying heat. The phase goes into a new phase, say from liquid to vapor by breaking its intermolecular bonds.

At the point of the phase change the phase has maximum enthalpy, for example for water to steam phase change the liquid water called saturated water has a maximum enthalpy 419 kJ/ kg . It has maximum entropy and minimum free energy for two phases to go into thermodynamic equilibrium. This equilibrium is maintained during the phase change process when the dryness fraction, X = 0 at the start point of phase change reaches X = 1 at the end point and saturated water at 100 degc/1 bar becomes saturated steam at 100 degc / 1.

During the entire phase change process temperature and pressure remain constant.

Now let us look at an adiabatic process.

In an adiabatic process, U = W ( U internal energy and W work) keep compensating each other. In adiabatic expansion, U supplies energy to W. In adiabatic compression, W supplies energy to U. in either case U changes and therefore, the temperature is not constant internally in an adiabatic process. Therefore, an adiabatic process does not meet the requirements for phase change.

要查看或添加评论,请登录

社区洞察

其他会员也浏览了