Engineering guide
The EGAS module calculates the physical properties of gaseous natural gas as a function of pressure and temperature: density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, Prandtl number, coefficient of thermal expansion, thermal diffusivity, specific enthalpy, isentropic exponent and compressibility factor. In addition, the molar mass, the specific gas constant and the standard density at 0 °C and 1.01325 bar are reported.
Since natural gas is not a pure substance, the module works with the gas composition: the two characteristic qualities natural gas L and natural gas H are built in; alternatively, the mole fractions of methane, ethane, propane, butane, nitrogen and carbon dioxide can be entered freely. This also allows deviating grid gases or process gases to be represented. The properties are calculated for two state points simultaneously, for example the inlet and outlet of an apparatus.
These property data are needed wherever natural gas flows or is heated: when designing natural gas preheaters and heat exchangers (Prandtl number, thermal conductivity and viscosity for the Nusselt correlations), for pressure drop and flow calculations (density, compressibility factor, isentropic exponent), and for compressor and control loop design. If you want to calculate natural gas properties, this module delivers consistent input data for the downstream heat transfer and fluid flow modules.
Scope
- Properties of natural gas
Typical workflow
Select the gas composition
Either one of the built-in standard qualities natural gas L or natural gas H is selected, or the composition of methane, ethane, propane, butane, nitrogen and carbon dioxide is entered freely. From this, the molar mass and the specific gas constant of the mixture follow.
Define the state points
For up to two states – typically inlet and outlet – pressure and temperature are specified. All properties are calculated for both points in parallel, so mean values for the equipment design can be read off directly.
Determine real gas behavior
The module determines the compressibility factor of the mixture at the respective state. It corrects the density relative to the ideal gas law and enters the enthalpy and the isentropic exponent – at higher pressures, this correction is no longer negligible for natural gas.
Calculate caloric and transport properties
The specific heat capacity, thermal conductivity, and dynamic and kinematic viscosity are determined as mixture values; from these follow the derived quantities Prandtl number and thermal diffusivity as well as the coefficient of thermal expansion.
Hand results over to subsequent calculations
Standard density, enthalpy difference and transport properties are available for heat duty balances, heat transfer and pressure drop calculations and can be reused in linked modules.
Frequently asked questions
What is the difference between natural gas L and natural gas H?
H-gas (high calorific) has a high methane content and only little inert gas, while L-gas (low calorific) contains significantly more nitrogen and sometimes carbon dioxide, which noticeably changes the calorific value and density. For equipment design, this mainly affects the molar mass, gas constant, density and heat capacity. If the actual grid gas analysis is available, it should be used via the free input of the component fractions rather than the standard qualities.
When must the compressibility factor be taken into account?
At atmospheric pressure, natural gas deviates from ideal gas behavior by only a few parts per thousand. With increasing pressure, however, the compressibility factor drops well below 1 – at typical transmission grid pressures of 40 to 80 bar it lies in the range of about 0.85 to 0.95, depending on temperature and composition. Density, volume flow conversion and enthalpy then become significantly wrong under an ideal-gas assumption; the real gas correction is therefore mandatory from moderate pressures upward for pressure drop and compressor calculations.
What is the standard density at 0 °C and 1.01325 bar used for?
In gas supply engineering, gas quantities are almost always specified and billed as standard volume flow in Nm³/h. The standard density links this standard volume flow to the mass flow and, via the operating state, to the actual operating volume flow, which governs flow velocity and pressure drop. Note that besides the physical standard state (0 °C) there are other reference states (e.g. 15 °C) – the reference basis must match the quantity definition of the project.
Can the module also be used for biogas or other gas mixtures?
The free component input covers methane, ethane, propane, butane, nitrogen and carbon dioxide. Upgraded biomethane can be represented well with this. Raw biogas with high CO2 contents formally lies within the input range but often contains additional components such as water vapor or hydrogen sulfide, which the module does not cover; for such gases the results should only be used as an approximation. Hydrogen admixtures are not supported.
Worked example
For a natural gas H with a given composition, the molar mass, the specific gas constant and the standard density (0 °C, 1.01325 bar) are to be determined – the base quantities from which the module derives all further properties. A worked example of a natural gas property calculation.
Given
| Methane CH4 (mole fraction) | 93.0 % |
|---|---|
| Ethane C2H6 | 3.0 % |
| Propane C3H8 | 1.3 % |
| Butane C4H10 | 0.6 % |
| Nitrogen N2 | 1.0 % |
| Carbon dioxide CO2 | 1.1 % |
| Standard state | 0 °C; 1.01325 bar |
Solution
Molar mass of the mixture
The molar mass follows as the mole-fraction-weighted mean of the component molar masses (CH4: 16.043; C2H6: 30.069; C3H8: 44.096; C4H10: 58.122; N2: 28.014; CO2: 44.010 kg/kmol):
M = 0.93 · 16.043 + 0.03 · 30.069 + 0.013 · 44.096 + 0.006 · 58.122 + 0.010 · 28.014 + 0.011 · 44.010
M = 14.920 + 0.902 + 0.573 + 0.349 + 0.280 + 0.484 = 17.51 kg/kmol
Specific gas constant
With the universal gas constant R = 8,314.462 J/(kmol·K):
Rs = R / M = 8,314.462 / 17.508 = 474.9 J/(kg·K)
Standard density
At the standard state (Tn = 273.15 K; pn = 101,325 Pa), the compressibility factor of natural gas is close to 1, so the ideal gas equation provides a very good approximation:
ρn = pn / (Rs · Tn) = 101,325 / (474.9 · 273.15) = 0.781 kg/m³
This lies in the typical range for H-gas (standard density approx. 0.78–0.84 kg/m³).
Result
| Molar mass M | 17.51 kg/kmol |
|---|---|
| Specific gas constant Rs | 474.9 J/(kg·K) |
| Standard density ρn | 0.781 kg/m³ |
Technical content source: LV-Solved-Problems-Content 628d313247e7