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D4784-23 – Standard Test Method Technical Guide
ASTM D4784-23 establishes a rigorous framework for calculating the density of saturated Liquefied Natural Gas (LNG) mixtures under defined conditions. This specification is essential for ensuring uniformity and high precision in LNG density predictions, directly supporting critical applications such as custody transfer, inventory control, and process engineering. The model relies on the extended corresponding states (ECS) method and provides a guaranteed uncertainty within stated operational limits.
📐 Scope and Composition Limits
The specification applies strictly to saturated LNG mixtures within a temperature range of 90 K to 120 K. To achieve the model’s high accuracy, the molar composition of the LNG must conform to specific limits. A fundamental assumption of the model is the complete absence of any hydrocarbons with a carbon number of six or greater (C6+).
| 🟦 Component | 📏 Maximum Mole Fraction |
|---|---|
| Methane | ≥ 60 % (minimum) |
| Nitrogen | < 4 % |
| n-Butane | < 4 % |
| i-Butane | < 4 % |
| Pentanes | < 2 % |
| C6+ Hydrocarbons | Not present |
⚙️ Extended Corresponding States Method
The standard employs the extended corresponding states (ECS) method. In this approach, an equation of state for methane serves as the reference fluid. The equation was specially modified to define a realistic vapor-liquid phase boundary down to 43 K, a necessary adjustment for modeling the low reduced temperatures of heavier hydrocarbons without altering its performance above the triple point of methane. Transformation functions fii,o and hii,o, based on the ratios of critical temperatures (Tc) and critical volumes (Vc), map each target component to the methane reference. The model defines the compressibility factor (Z) and Gibbs free energy (G) for the mixtures through this correspondence.
💡 Methodological Note: The modification of the methane equation of state was a critical development. It allowed the standard to cover the reduced temperatures of heavier components like butanes and pentanes without sacrificing accuracy for the primary methane component.
📊 Accuracy and Key Parameters
The headline performance metric of ASTM D4784-23 is its density calculation uncertainty. When the mixture adheres perfectly to the specified composition boundaries and temperature range, the model predicts the density of the saturated liquid within ±0.1 % of the true value. Strict usage of SI units (specifically Kelvin for temperature and Pascals for pressure) is required.
| ⚡ Parameter | 🎯 Specification |
|---|---|
| State of Fluid | Saturated Liquid |
| Temperature Range | 90 K to 120 K |
| Density Uncertainty | ±0.1 % |
| Reference Fluid | Methane |
| Unit System | SI (Standard International) |
⚠️ Strict Compliance Required: The ±0.1 % uncertainty is only valid within the defined temperature and composition windows. Specifically, the model is not valid for mixtures containing C6+ hydrocarbons, more than 2 % pentanes, or more than 4 % nitrogen or butanes.
❓ Frequently Asked Questions
🔍 What is the permissible temperature range for the standard?
The standard is applicable to saturated LNG mixtures operating within a temperature range of 90 K to 120 K.
💡 Why was methane selected as the reference fluid?
Methane is the dominant component in LNG, typically comprising 60 % or more of the mixture. Its well-characterized equation of state provides a stable and accurate baseline for the extended corresponding states model.
⚡ What is the declared uncertainty of the density calculation?
When all compositional and temperature constraints are met, the estimated uncertainty of the density calculation is ±0.1 % of the true value.
📌 Are there specific restrictions on heavier hydrocarbons?
Yes. Pentanes are limited to less than 2 % of the molar composition.