Critical temperature and critical pressure
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For a gas, taking propane as an example here, its critical temperature and critical pressure can be found to be: TC=369.8 K, PC=4.246 MPa. But I want to know how to obtain its critical pressure at some temperature below TC=369.8 (K)?
It can be found in textbooks on physical chemistry or chemical engineering thermodynamics.
Check out \"Thermodynamics of Chemical Engineering\"; it contains P-V-T diagrams of fluids, and from these diagrams the critical pressure corresponding to the critical temperature can be determined
The relevant calculation methods can be found in the Handbook of Physical Properties
Critical pressure: the pressure at which a substance is in a critical state. This pressure is the saturated vapor pressure of the liquid at its critical temperature. The pressures at other temperatures are not critical pressures. It can be calculated using the Antoine equation.
There is only one critical temperature and one critical pressure
I have no choice but to consult the manual; moreover, the critical temperature and critical pressure are not obtained through calculations – they are fundamental physical properties. Just like the density of an object, once the object is determined, its density remains constant.
The critical temperature and critical pressure are the only ones, right? Have I remembered them wrong?
The pressure at which a critical-pressure substance is in a critical state. This pressure is the saturated vapor pressure of the liquid at its critical temperature ； The pressures at other temperatures are not critical pressures. It can be calculated using the Antoine equation; therefore, there is only one critical pressure.
The critical temperature and critical pressure are unique
Find a book on chemical thermodynamics; you can use tables or perform calculations using formulas to get the values For the same substance, a higher saturation pressure corresponds to a higher saturation temperature. Increasing the pressure can raise the liquefaction temperature, making it easier for the gas to liquefy. That is, at a certain temperature, it can be liquefied by increasing the pressure. However, for each substance, once the temperature exceeds a certain value, it cannot be liquefied no matter how high the pressure is increased. This temperature is called the “critical temperature”. The critical temperature is the highest temperature at which a substance can be liquefied. The liquefaction pressure corresponding to the critical temperature is called the critical pressure. Different substances have different critical temperatures and critical pressures, as shown in Table 7. Table 7: Critical temperatures and critical pressures of certain substances. Substance name: Air, O2, N2, H2, ONH3, CO2, H2. Critical temperature/°C: -140.65 to -140.75, -118.40, -146.90, 374.15, 132.40, 31.00, -239.60. Critical pressure/MPa: 3.868 to 3.876, 5.079, 3.394, 22.565, 11.580, 7.530, 1.320. At the critical temperature and critical pressure, there is no significant difference between the gas and liquid states ； Above the critical pressure, when the temperature drops below the critical temperature, everything turns into a liquid; there is no phase transition stage nor phase transition latent heat. The vaporization process in the reverse direction is the same. In the internal compression process, liquid oxygen is compressed to the desired pressure within the unit, and then reheated and vaporized in a high-pressure heat exchanger. If the compression pressure of liquid oxygen is below the critical pressure (for example, the pressure of oxygen used in steelmaking is 3.0 MPa), then during the gasification process in the heat exchanger, there is a phase of gasification during which heat is absorbed and the temperature remains constant, followed by a superheating phase during which the gas temperature rises ； If the compression pressure of liquid oxygen is higher than the critical pressure (for example, 6.0 MPa or higher for oxygen used in the chemical industry), then during the vaporization process in the heat exchanger, there is no vaporization stage at a constant temperature. This will affect the heat transfer performance of the high-pressure heat exchanger, and it needs to be taken fully into account during design.
The critical pressure is the pressure at which a substance is in a critical state; it corresponds to the saturated vapor pressure of a liquid at its critical temperature, and it is unique for a given substance.
You can check the manual or use formulas for calculations, after all, the information in the manual is also the result of calculations.
Material science or other chemical engineering handbooks contain information on this topic. It is generally looked up using a diagram.