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D3956-23 – Standard Test Method Technical Guide
The ASTM D3956-23 standard provides authoritative thermophysical property tables for methane, specifically developed for accurate process design and operations. Covering a temperature range from 90 K to 600 K at pressures up to 30 MPa, these tables are essential for calculating pressure-volume-temperature (PVT), thermodynamic, and transport properties. Developed by the National Institute of Standards and Technology (NIST) using the REFPROP version 10.0 database, the standard defines three distinct sets of tables: saturation properties for liquid and vapor phases, and a comprehensive set of thermodynamic data along isobars.
📐 Scope and Applicability
This standard is directly applicable to pure gaseous methane but is designed to provide a critical foundation for modeling mixtures containing methane, such as liquefied natural gas (LNG). The three primary data tables specified within the standard are:
- Liquid at Vapor-Liquid Equilibrium: Saturation properties of liquid methane.
- Vapor at Vapor-Liquid Equilibrium: Saturation properties of vapor methane.
- Properties Along Isobars: Selected T, p points across multiple isobars for the equilibrium phase.
💡 Source Database: The tables were generated using the NIST Standard Reference Database 23 (REFPROP), Version 10.0. This tool allows users to access a wider variety of properties and unit systems beyond the SI units provided in the standard text.
📊 Key Thermophysical and Transport Properties
The standard tabulates eight essential properties for each phase condition. These cover thermodynamic state variables, caloric properties, and transport characteristics.
| 🟦 Property | 📏 Symbol | 📐 SI Unit | 🎯 Role |
|---|---|---|---|
| Molar Density | ρ | mol·L⁻¹ | PVT Relationship |
| Molar Enthalpy | H | J·mol⁻¹ | Energy Balance |
| Molar Entropy | S | J·K⁻¹·mol⁻¹ | Energy Balance |
| Constant Volume Heat Capacity | Cv | J·K⁻¹·mol⁻¹ | Caloric Property |
| Constant Pressure Heat Capacity | Cp | J·K⁻¹·mol⁻¹ | Caloric Property |
| Speed of Sound | c | m·s⁻¹ | Acoustic Property |
| Viscosity | η | µPa·s | Transport Property |
| Thermal Conductivity | λ | mW·m⁻¹·K⁻¹ | Transport Property |
⚙️ Reference States and Data Origin
Accurate calculation of enthalpy (H) and entropy (S) from an equation of state requires defined reference states. D3956-23 establishes specific reference conditions for generating the standard tables, which have been updated in this version.
⚠️ Updated Reference State: The standard explicitly adopts a new reference state for this edition. At a base condition of 298.15 K and 0.101325 MPa, the molar enthalpy is set to 10018 J·mol⁻¹ and the molar entropy is set to 186.266 J·K⁻¹·mol⁻¹.
The standard molar mass of methane used in these calculations is 16.043 g·mol⁻¹. This value is critical for converting between mass-based and mole-based property units.
| ⚡ Reference Parameter | 📐 Specified Value |
|---|---|
| Temperature | 298.15 K |
| Pressure | 0.101325 MPa |
| Molar Enthalpy (H) | 10018 J·mol⁻¹ |
| Molar Entropy (S) | 186.266 J·K⁻¹·mol⁻¹ |
| Molar Mass of Methane | 16.043 g·mol⁻¹ |
❓ Frequently Asked Questions
🔍 What is the operational temperature and pressure range covered?
The standard provides reliable data for engineering calculations across a range from 90 K (-183 °C) to 600 K (327 °C) and at pressures reaching 30 MPa (4351 psi).
💡 What transport properties are included in the tables?
Beyond thermodynamic properties, the standard tabulates viscosity (η) in µPa·s, thermal conductivity (λ) in mW·m⁻¹·K⁻¹, and the speed of sound (c) in m·s⁻¹.
⚡ Why were the reference states updated in this version?
The reference state for enthalpy and entropy has been updated to align with the latest formulations in the NIST REFPROP database. The new standard state is defined at 298.15 K and 0.101325 MPa.
📌 Is the standard applicable only to pure methane?
Yes, the tables are developed specifically for pure methane. However, the standard explicitly states they are expected to find substantial use in mathematical models for mixtures containing methane, providing a critical baseline for LNG and natural gas property calculations.