ISO 6578 - LNG Density Calculation

ISO 6578 provides methods for calculating the density of liquefied natural gas (LNG) from composition and temperature.

Table of Contents

Overview

Standard: ISO 6578:2017 - Refrigerated hydrocarbon liquids — Static measurement — Calculation procedure

Purpose: Calculate LNG density for custody transfer and inventory management.

Scope:

Class: Standard_ISO6578

Calculation Method

Principle

LNG density is calculated from:

  1. Ideal mixing - Pure component molar volumes
  2. Excess volume correction - Klosek-McKinley correction factors

Density Equation

\[\rho = \frac{M_{mix}}{V_{mix}}\]

where: \(V_{mix} = \sum_i x_i V_i + \Delta V_{correction}\)

Klosek-McKinley Correction

\[V_{mix} = \sum_i x_i V_i - k_1 x_{N_2} - k_2 x_{CH_4}\]

where:

Implementation

Constructor

import neqsim.standards.gasquality.Standard_ISO6578;

// Create standard
Standard_ISO6578 iso6578 = new Standard_ISO6578(thermoSystem);

Key Methods

Method Description
calculate() Perform density calculation
getValue("density", "kg/m3") Get density in kg/m³
useISO6578VolumeCorrectionFacotrs(boolean) Toggle ISO 6578 vs alternative factors

Internal Data

The class includes tabulated data for:

Usage Examples

Basic LNG Density Calculation

import neqsim.thermo.system.SystemSrkEos;
import neqsim.standards.gasquality.Standard_ISO6578;

// Create LNG composition at storage temperature
SystemInterface lng = new SystemSrkEos(273.15 - 162, 1.01325);  // -162°C
lng.addComponent("methane", 0.9200);
lng.addComponent("ethane", 0.0500);
lng.addComponent("propane", 0.0180);
lng.addComponent("n-butane", 0.0040);
lng.addComponent("i-butane", 0.0030);
lng.addComponent("nitrogen", 0.0050);
lng.setMixingRule("classic");

// Calculate density
Standard_ISO6578 iso6578 = new Standard_ISO6578(lng);
iso6578.calculate();

double density = iso6578.getValue("density", "kg/m3");
System.out.printf("LNG Density at %.1f°C = %.2f kg/m³%n", 
    lng.getTemperature() - 273.15, density);

Density at Different Temperatures

// Temperature sweep
double[] temperatures = {-165, -162, -160, -155, -150};  // °C

for (double T : temperatures) {
    lng.setTemperature(T + 273.15);
    iso6578.calculate();
    double rho = iso6578.getValue("density", "kg/m3");
    System.out.printf("T = %.0f°C: ρ = %.2f kg/m³%n", T, rho);
}

Rich LNG (High Ethane/Propane)

// Rich LNG composition
SystemInterface richLNG = new SystemSrkEos(273.15 - 162, 1.01325);
richLNG.addComponent("methane", 0.8500);
richLNG.addComponent("ethane", 0.0900);
richLNG.addComponent("propane", 0.0400);
richLNG.addComponent("n-butane", 0.0100);
richLNG.addComponent("nitrogen", 0.0100);
richLNG.setMixingRule("classic");

Standard_ISO6578 standard = new Standard_ISO6578(richLNG);
standard.calculate();

double density = standard.getValue("density", "kg/m3");
System.out.printf("Rich LNG Density = %.2f kg/m³%n", density);

// Rich LNG has higher density due to heavier components

Comparing Correction Factor Options

// Using ISO 6578 correction factors (default)
Standard_ISO6578 iso_factors = new Standard_ISO6578(lng);
iso_factors.useISO6578VolumeCorrectionFacotrs(true);
iso_factors.calculate();
double rho_iso = iso_factors.getValue("density", "kg/m3");

// Using alternative correction factors
Standard_ISO6578 alt_factors = new Standard_ISO6578(lng);
alt_factors.useISO6578VolumeCorrectionFacotrs(false);
alt_factors.calculate();
double rho_alt = alt_factors.getValue("density", "kg/m3");

System.out.printf("ISO 6578 factors: %.2f kg/m³%n", rho_iso);
System.out.printf("Alternative factors: %.2f kg/m³%n", rho_alt);

Correction Factors

Klosek-McKinley Correction Factor Tables

The implementation includes two sets of correction factors:

ISO 6578 Factors

Temperature range: 93.15 K to 133.15 K (-180°C to -140°C) Molar mass range: 16 to 30 g/mol

Temperatures: {93.15, 98.15, 103.15, 108.15, 113.15, 118.15, 123.15, 128.15, 133.15} K
Molar Masses: {16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30} g/mol

Alternative Factors

Temperature range: 105 K to 135 K Molar mass range: 16 to 25 g/mol

Interpolation

Bicubic interpolation is used for:

Linear interpolation for pure component molar volumes.

Component Data

Supported Components

Component Formula Molar Volume Data
Methane CH₄ Yes
Ethane C₂H₆ Yes
Propane C₃H₈ Yes
n-Butane C₄H₁₀ Yes
i-Butane C₄H₁₀ Yes
n-Pentane C₅H₁₂ Yes
i-Pentane C₅H₁₂ Yes
n-Hexane C₆H₁₄ Yes
Nitrogen N₂ Yes

Molar Volume Data

Pure component molar volumes at reference temperatures:

Temperatures: {93.15, 98.15, 103.15, 108.15, 113.15, 118.15, 123.15, 128.15, 133.15} K

Example - Methane (dm³/mol):
{0.035771, 0.036315, 0.036891, 0.037500, 0.038149, 0.038839, 0.039580, 0.040375, 0.041237}

Typical Results

Lean LNG (High Methane)

Composition Value
Methane 95%
Ethane 3%
Others 2%
Density at -162°C 425-435 kg/m³

Standard LNG

Composition Value
Methane 90-92%
Ethane 5-6%
Propane 2%
Others 1-2%
Density at -162°C 440-460 kg/m³

Rich LNG (High C2+)

Composition Value
Methane 85%
Ethane 9%
Propane 4%
Others 2%
Density at -162°C 470-490 kg/m³

Accuracy Considerations

Temperature Sensitivity

LNG density is strongly temperature dependent:

Composition Sensitivity

Uncertainty

Typical uncertainty: ±0.1% for well-characterized LNG

References

  1. ISO 6578:2017 - Refrigerated hydrocarbon liquids — Static measurement — Calculation procedure
  2. Klosek, J., McKinley, C. (1968). Densities of Liquefied Natural Gas and of Low Molecular Weight Hydrocarbons. Proceedings of the First International Conference on LNG.
  3. GIIGNL Custody Transfer Handbook (5th Edition)