What is Compressibility factor: Definition and 11 Discussions

In thermodynamics, the compressibility factor (Z), also known as the compression factor or the gas deviation factor, is a correction factor which describes the deviation of a real gas from ideal gas behaviour. It is simply defined as the ratio of the molar volume of a gas to the molar volume of an ideal gas at the same temperature and pressure. It is a useful thermodynamic property for modifying the ideal gas law to account for the real gas behaviour. In general, deviation from ideal behaviour becomes more significant the closer a gas is to a phase change, the lower the temperature or the larger the pressure. Compressibility factor values are usually obtained by calculation from equations of state (EOS), such as the virial equation which take compound-specific empirical constants as input. For a gas that is a mixture of two or more pure gases (air or natural gas, for example), the gas composition must be known before compressibility can be calculated.
Alternatively, the compressibility factor for specific gases can be read from generalized compressibility charts that plot


{\displaystyle Z}
as a function of pressure at constant temperature.
The compressibility factor should not be confused with the compressibility (also known as coefficient of compressibility or isothermal compressibility) of a material, which is the measure of the relative volume change of a fluid or solid in response to a pressure change.

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  1. A

    Fluid mechanics - Additional liquid capacity due to compression

    A cylindrical tube (diameter = D, width = L) is completely filled with a liquid (density = ρ). A pump pressurizes the system with a pressure P. Consequently, 1) the solid tube is compressed and deformed according to Hooke's law (σ = ε.E), and 2) the liquid is compressed and deformed, following...
  2. mohamed_a

    I Thermodynamic constant -- misunderstanding

    I was reading about thermodynamics in my textbook wheni came across the following thermodynamics constants: However, i don't understand why did we define 1/V inthe constants. What is the point in doing this?
  3. FranzSC

    B NIST database and compressibility factor Z

    There's a huge volume of data on NIST database: https://webbook.nist.gov/cgi/fluid.cgi?T=293.15&PLow=10&PHigh=1000&PInc=10&Applet=on&Digits=5&ID=C7727379&Action=Load&Type=IsoTherm&TUnit=K&PUnit=bar&DUnit=mol%2Fl&HUnit=kJ%2Fmol&WUnit=m%2Fs&VisUnit=uPa*s&STUnit=N%2Fm&RefState=DEF I'm interested...
  4. L

    Thermodynamics Compressibility Factor

    Homework Statement Please consider ethylene at 152oF and 126 atm. Please determine the molar volume (ft3/lbmole) if Z is determined by Corresponding States Theory. Homework Equations Z=PVm/RmT Vm= Molar volume R=Rm/M M= molecular weight Rm=1545(ft*lbf)/(lbmol*oR) Zc=(Pcvc)/(R*Tc) Tc=283 K...
  5. HethensEnd25

    Compressibility factor for an Isotherm

    Homework Statement Why is it that when considering the compressibility factor of a gas Z we see the value decrease at first and increase as value for Pr increases for an isotherm Tr=1? Homework Equations z=1+(Pb/RT)-(a/VRT) The Attempt at a Solution It is my understanding that as Pr is low...
  6. S

    Compressibility factor and van der Waals equation for temp

    The pressure exerted on the walls of the container by a real gas is less compared to an ideal gas. This is due to the attractive forces of the gas pulling the molecules back towards the rest of the gas molecules. However, there is also a relationship whereby at lower temperatures, the z is even...
  7. S

    Question about compressibility factor

    At low temperatures, z falls below 1 and the reason for that is because the intermolecular interactions cause the pressure exerted to be lesser than expected. PVm/RT=z and since P is less than expected z drops below 1. However, as the pressure increases z increases to be above 1 because as P...
  8. A

    Compressibility Factor Pressure For Calculating Fan Power

    What is the correct type of pressure (static or total) used in the compressibility factor, KP, when calculating fan power? Howden's Fan Engineering book seems to indicate total pressures should be used, but I also have a PDF from Howden that indicates static pressures should be used. Online...
  9. M

    Volume using Compressibility Factor

    Homework Statement Find the volume of 2 kg of ethylene at 270 K, 2500 kPa using Z Homework Equations Method to Solve for Z, Using Tr and Pr PV = ZnRT The Attempt at a Solution Tr = 0.9561 Pr = 0.496 Z found to be approx. 0.75 R given on a table at 0.2964 kJ/kg K From...
  10. A

    Finding the compressibility factor (Z)

    Homework Statement Determine the volume, in m^3, occupied by 20 kg of hydrogen (H2) at 1170 kPa, 2220°C. Homework Equations Z=pv/rt, Pr=P/Pc, Tr=T/Tc, and for hydrogen M = 2.016 (kg/kmol) Tc = 33.2 (K) Pc = 13.0 bar Zc=pc*vc/(RTc) The Attempt at a Solution I know if I find Z then...
  11. N

    [Thermo] Derivation of compressibility factor vs reduced pressure

    Homework Statement derivation of compressibility factor vs. reduced pressure I am supposed to derive the graph by solving equations Homework Equations Van der Waals equation of state compressibility factor, Z = (Pv)/(RT) reduced pressure = P/critical pressure Z = f(Tr, Pr) The...