Phase Diagram of Water: Definition and Explanation
What is Phase Diagram?
The phase diagram is a graphical representation of phase change of a substance under different temperature and Pressure conditions. Phase diagram of water is a pressure-temperature diagram, that represents various phases of water like ice, water, and steam under various condition.
In the above diagram, X-axis represents Temperature and Y-axis represents Pressure. Blue, Green and Yellow Colour represents three different phases of water. The first curve becomes the boundary between the solid phase and the gas phase. The second curve is the boundary between the solid phase and the liquid, whereas the third curve becomes the boundary between the liquid phase and the gas. This third curve is known as the saturated line. The phase of water at this line is saturated steam. Along the saturation line, regardless of the magnitude of pressure and temperature, water and steam present in equilibrium. End of the first curve is the Triple point of water. At the Triple point, water, ice, and steam co-exist together. This condition is at a pressure of 0.61 kPa and 0.010°C. Seen in the diagram above, below the triple point, water has no liquid phase. Under this triple point, the solid water (ice) will instantly evaporate into the gas if there is a rise in temperature at constant pressure. The phase of water vapor can be classified again into three, saturated steam, superheated steam, and supercritical steam.
Above the triple point, there are two branching curves with each function. One curve stays between solid and liquid phases and the other curves constrain between the liquid phase and the gas. At the end of the liquid-gas curve, there is a point we know as a critical point. The critical point is a point that becomes the end of the liquid and gas phase equilibrium curve so that it can be in the same condition of pressure and temperature. The critical point of water is at 22.1 MPa and 374°C.
Water vapor at pressures and temperatures over critical points cannot be condensed only by increasing the pressure alone. Or another example, if we heat water at a constant pressure of 25 MPa, liquid water will not experiencing saturated steam phase — which is a mixture of water and water vapor — but will instantaneously turn the phase into supercritical vapors. This concept is the basis for the use of supercritical boilers in modern steam power plants.
Phase diagram of Water
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions at which thermodynamically distinct phases occur and coexist at equilibrium.
What is Saturated Steam?
As explained above Saturated steam is a condition in which water vapor has an equilibrium pressure and temperature with liquid water. Saturated steam is a wet steam, which still mixed with liquid water-phase molecules. This saturation vapor begins to form just as the water reaches its boiling point until all the energy from latent heat is absorbed by water. While all latent heat has been absorbed by water, and the amount of vapor phase has reached almost 100% of its, that is the end of the vapor phase of saturation. The process of reaching almost 100% of the vapor phase occurs at stable pressure and temperature. In the phase diagram of water, the saturated steam phase is shown as a line (between green and yellow color) of saturation curves which is the boundary between liquid water and superheated steam.
The phase of saturated steam can only form along the saturated curve. The lower limit of the saturated curve is the triple point, while the upper limit of the curve is the critical point. Water in more than triple-point conditions will not experience a phase of saturated steam. Water that has a pressure above 22.1 kPa, if it continues to be heated will immediately turn the phase into supercritical steam.
The mixture between water vapor and liquid water in the saturated steam can be determined in amount by using a saturated steam diagram. This diagram uses pressure as the Y-axis and the enthalpy as the X-axis. This saturated steam diagram is made of a curve. Half of the curve from the lowest point to the top is called the saturated water curve. This part curve becomes the boundary between liquid water with the saturated steam phase. For the right curve from the top of the curve to the lowest point is called the saturated steam curve. This curve becomes the boundary between the phase of saturated steam and superheated steam phase. Right at the vertex of the curve is a critical point, the same point as the critical point in the phase diagram of water.
Since the saturated steam is under constant pressure, a certain amount of saturated steam is represented by a horizontal straight line connecting a point on the saturated water curve to another point on the saturated steam curve. The point on the saturated water curve (hf) shows the enthalpy value of saturated water, ie how much heat energy required for water at pressure P per one unit of mass can reach saturated water. While the point on the saturated steam curve (hg) is the total enthalpy value required so that the water reaches 100% of steam.
The simple relationship is:
hg — hf = hfg
Where:
hf = enthalpy saturated water
hg = enthalpy saturated steam
hfg = difference of enthalpy required saturated water to achieve saturated steam
In other cases, the enthalpy value given to water is not as large as hg, ie only by hmix. The hmix point is anywhere along the horizontal line. In this case the saturation vapor is a mixture of vapor with water whose ratio can be easily determined using the following equation:
So:
hmix = hf + x . hfg
Where:
x = comparison of the amount of water in the overall vapor mixture of saturation
hmix = enthalpy mixture
What is Superheated Steam?
Superheated steam is a phase of water that has passed through the saturation phase by absorbing more thermal energy so that the entire water fluid has a pure gas phase. Superheated steam contains high thermal energy, and used as an energy medium of the power plant steam turbine. The dry and pure phase characteristics of this have become the absolute requirement of steam before it can enter the steam turbine. This is because superheated steam is able to avoid the damage caused by the erosion of wet moisture as in saturated steam.
Water Liquid Phase change to Superheated Steam
Liquid water can turn the phase into superheated steam after crossing the saturation curve line.
Case1: First, with fixed pressure, the environmental temperature is increased.
Case2: Second at a fixed temperature, the environmental pressure is lowered.
Case3: Room pressure is lowered along with rising room temperature.
But all these processes must meet one condition, the line of change of environmental conditions must cut the saturation curve.
Solid Water Phase (Ice) change to Superheated Steam
Ice can directly change the phase to dry superheated steam without having to pass through the liquid phase, and vice versa.
Superheated steam can directly change the phase to ice without having to pass through the liquid phase. This change of phase must be at a lower pressure than the triple point, ie 0.61 kPa, or 0.006 atm, almost vacuum pressure.
When the cloud above the Earth’s atmosphere which has vacuum pressure, it cools rapidly so that the water vapor changes instantly into ice, We call it sleet.
Supercritical Steam Phase Change becomes Superheater Steam
Supercritical steam can turn the phase into superheated steam by lowering the pressure.
Supercritical steam has pressure and working temperature above the critical point, which is more than 22.1 MPa and 374 ° C.
Thus, a certain amount of supercritical steam is at 25 MPa and a temperature of 600°C decreases the pressure to 18 MPa, the steam will turn into superheated steam.
What is Supercritical Steam?
Supercritical steam is a water phase condition above the critical point of water, which is 22.1 MPa and 374°C. There are two characteristics of supercritical steam: water has no latent heat value, as well as the same volume-specific value between water and supercritical steam. Supercritical steam phase has an indistinguishable phase between liquid nor gas. Supercritical steam is used at modern steam turbine power plants which are pursuing high efficiency. The main challenge of using this supercritical steam is in the design of boiler tube materials. The boiler tube should be very resistant to thermal stress causing by the generation process of supercritical steam.
Beyond the critical point (>647.096 K, >22.064 MPa) in the liquid-vapor space on the right. At the critical point, there is about 30% free monomeric H2O molecules and about 17% hydrogen bonding with other supercritical fluids. supercritical water has no surface tension with its gas or liquid phase or any other supercritical phase as no such interfaces exist. Above 647.096 K, water vapor cannot be liquefied by increasing pressure.
Properties of Supercritical Steam:
The properties of supercritical water are very different from ambient liquid water.
- Supercritical water is a poor solvent for electrolytes. However, it is such an excellent solvent for non-polar organic molecules, due to its low relative permittivity (dielectric constant) and poor hydrogen bonding, that many are completely miscible.
- Viscosity and dielectric both decrease substantially whereas auto-dissociation increases substantially. The physical properties of water close to the critical point (near-critical) are particularly strongly affected by density.
- An extreme density fluctuation around the critical point causes opalescent turbidity. At the critical point, only one phase exists. The heat of vaporization is zero.
Originally published at Merrykit
