Skip to main content

Fluid model

To calculate the energy usage related to compression of a natural gas, information about the composition is needed, i.e. which components it consists of and the (mole) fraction of each. Typical components for natural gas are alkanes such as methane, ethane, propane, butane, pentane, hexane in addition to water, nitrogen and carbone dioxide. Alkanes with seven or more carbon atoms may occur, but these are often just part of the liquid (oil) phase and not significant in dry gas compression.

The composition can either be selected from a set of predefined compositions available within eCalc, or the mole fraction of each component can be user specified directly. This is controlled through the FLUID_MODEL_TYPE keyword.

As the fluid is going through the compressor in a fluid dynamic process, the enthalpy changes, resulting in a new state with increased pressure and temperature, and decreased volume. To estimate these changes, an equation-of-state (EOS) model is used. The default EOS model in eCalc is SRK (Soave-Redlich-Kwong).

The GERG models (GERG 2008) are used to calculate enthalpy, gamma and density, whilst other properties such as molar mass is based on either SRK or PR.

Available EOS models

  • SRK (Soave-Redlich-Kwong)
  • PR (Peng-Robinson)
  • GERG_SRK
  • GERG_PR

This is controlled through the EOS_MODEL keyword. If the EOS_MODEL is not specified, it will be defaulted to SRK.

Fluid model using predefined composition​

If the FLUID_MODEL_TYPE is PREDEFINED the GAS_TYPE must be defined. Available gas types (with mole weights) are:

  • ULTRA_DRY (17.1 kg/kmol)
  • DRY (18.3 kg/kmol)
  • MEDIUM (19.4 kg/kmol)
  • RICH (21.1 kg/kmol)
  • ULTRA_RICH (24.6 kg/kmol)

If no GAS_TYPE is specified, it will be defaulted to MEDIUM.

Format​

MODELS:
  - NAME: <name of fluid model, for reference>
    TYPE: FLUID
    FLUID_MODEL_TYPE: PREDEFINED
    EOS_MODEL: <eos model>
    GAS_TYPE: <name of a predefined composition>

Examples​

Examples with predefined fluid

MODELS:
  - NAME: fluid_model_reference_name
    TYPE: FLUID
    FLUID_MODEL_TYPE: PREDEFINED
    EOS_MODEL: SRK
    GAS_TYPE: MEDIUM
MODELS:
  - NAME: fluid_model_reference_name
    TYPE: FLUID
    FLUID_MODEL_TYPE: PREDEFINED
    EOS_MODEL: PR
    GAS_TYPE: ULTRA_DRY

Example where EOS is defaulted to SRK and GAS_TYPE defaulted to MEDIUM

MODELS:
  - NAME: fluid_model_reference_name
    TYPE: FLUID
    FLUID_MODEL_TYPE: PREDEFINED

Fluid model with user-specified composition​

If the FLUID_MODEL_TYPE is COMPOSITION the COMPOSITION must be defined. The composition is specified by setting the mole fraction of each component. The available components are:

  • water
  • nitrogen
  • CO2
  • methane
  • ethane
  • propane
  • i_butane
  • n_butane
  • i_pentane
  • n_pentane
  • n_hexane

Setting the mole fraction for methane is required, all other components are optional and will be set to 0 if not specified. If methane is not part of your composition, simply put 0.0 for it.

It is not important that the fractions sum to one as they will be normalized by eCalc. It is the relative amount of each that will be important.

Format​

MODELS:
  - NAME: <name of fluid model, for reference>
    TYPE: FLUID
    FLUID_MODEL_TYPE: COMPOSITION
    EOS_MODEL: <eos model>
    COMPOSITION:
      water: <mole fraction>
      nitrogen: <mole fraction>
      CO2: <mole fraction>
      methane: <mole fraction, required>
      ethane: <mole fraction>
      propane: <mole fraction>
      i_butane: <mole fraction>
      n_butane: <mole fraction>
      i_pentane: <mole fraction>
      n_pentane: <mole fraction>
      n_hexane: <mole fraction>

Example​

MODELS:
  - NAME: <name of fluid model, for reference>
    TYPE: FLUID
    FLUID_MODEL_TYPE: COMPOSITION
    EOS_MODEL: srk
    COMPOSITION:
      water: 0.1
      nitrogen: 0.74373
      CO2: 2.415619
      methane: 85.60145
      ethane: 6.707826
      propane: 2.611471
      i_butane: 0.45077
      n_butane: 0.691702
      i_pentane: 0.210714
      n_pentane: 0.197937
      n_hexane: 0.368786