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In thermodynamics, the compressibility factor (Z), also known as the compression factor or the gas deviation factor, 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 .
Compressed natural gas (CNG) is a fuel gas mainly composed of methane (CH 4), compressed to less than 1% of the volume it occupies at standard atmospheric pressure.It is stored and distributed in hard containers at a pressure of 20–25 megapascals (2,900–3,600 psi; 200–250 atm), usually in cylindrical or spherical shapes.
The compressibility factor is defined as = where p is the pressure of the gas, T is its temperature, and is its molar volume, all measured independently of one another. In the case of an ideal gas, the compressibility factor Z is equal to unity, and the familiar ideal gas law is recovered:
These dimensionless thermodynamic coordinates, taken together with a substance's compressibility factor, provide the basis for the simplest form of the theorem of corresponding states. [1] Reduced properties are also used to define the Peng–Robinson equation of state, a model designed to provide reasonable accuracy near the critical point. [2]
Methane vapor pressure vs. temperature. ... Except where noted otherwise, data relate to Standard temperature and pressure. Reliability of data general note. References
The first term in the equation represents this high-pressure behavior. The second term corrects for the attractive force of the molecules to each other. The functional form of a with respect to the critical temperature and pressure is empirically chosen to give the best fit at moderate pressures for most relatively non-polar gasses. [11]
Where p is the pressure, T is the temperature, R the ideal gas constant, and V m the molar volume. a and b are parameters that are determined empirically for each gas, but are sometimes estimated from their critical temperature (T c) and critical pressure (p c) using these relations:
It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. [ 1 ] Other moduli describe the material's response ( strain ) to other kinds of stress : the shear modulus describes the response to shear stress , and Young's modulus describes the response to normal (lengthwise stretching) stress.