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D2597-10 – Standard Test Method Technical Guide
📊 Scope and Compositional Limits
ASTM D2597‑10 covers the gas chromatographic analysis of demethanized hydrocarbon liquid streams that contain nitrogen/air and carbon dioxide, including purity products such as ethane/propane mixtures. This test method is explicitly limited to mixtures containing less than 5 mol % of heptanes and heavier fractions. Typical compositional ranges addressed under this standard are listed in Table 1. Values are stated in SI units, with approximate imperial units given for information only.
| 🟦 Component | 📏 Concentration Range (mol %) |
|---|---|
| Nitrogen/Air | 0 – 5 |
| Carbon Dioxide | 0 – 10 |
| Ethane | 0 – 95 |
| Propane | 0 – 95 |
| n‑Hexane | 0 – 10 |
| Heptanes and Heavier | < 5 |
⚙️ Test Procedure and Column Switching
A fixed volume of liquid phase sample is isolated in a suitable inlet system and introduced onto the chromatographic column. With carrier flow in the forward direction, components from nitrogen/air through n‑hexane are individually separated. For samples containing a heptanes‑plus fraction, a switching valve reverses the carrier gas flow immediately after n‑hexane elutes, grouping the heavy end components into a single irregularly shaped peak. Purity mixes without heptanes and heavier are analyzed with no reverse flow.
| 🟦 Step | 📐 Description |
|---|---|
| 1 | Forward flow: Separate N₂/Air through n‑hexane |
| 2 | After n‑hexane elution, switch valve to reverse flow |
| 3 | Reverse flow: Group heptanes+ into single peak |
| 4 | For purity mixes: No reverse flow required |
⚠️ Caution: When a sample has a relatively large C₆+ or C₇+ fraction and precise results are important, the molecular weight (or other pertinent physical properties) of those fractions should be determined separately, since this test method does not provide for their determination.
🔬 Key Components and Calibration
Components are identified by comparing their retention times with calibration data obtained under identical operating conditions. The method incorporates several referenced standards for sampling and quality assurance, including Practice D1265, Test Method D2163, Practice D3700, Practice D6299, and Practice D6849. Heptanes and heavier fractions can be analyzed via reverse flow peak grouping or by using a precut column to elute these components first as a single peak.
💡 Tip: Calibration data must be obtained under the same operating conditions as the sample analysis to ensure accurate component identification and quantification.
❓ Frequently Asked Questions
🔍 What is the purpose of reversing the carrier gas flow?
Reverse flow is used immediately after n‑hexane elution to combine the heavy end components (heptanes and heavier) into a single peak, simplifying the analysis and reducing overall run time for samples containing these fractions.
💡 How are heptanes‑plus fractions handled when present?
They can be analyzed either by the reverse flow peak grouping method or by using a precut column that elutes the heavy components as a single peak first, as described in Section 1.2 of the standard.
⚡ Which ASTM standards are referenced for sampling?
The test method references Practice D1265 for manual sampling, Practice D3700 for floating piston cylinder sampling, and Practice D6849 for storage and use of LPG in sample cylinders, among others.
📌 What safety precautions apply to this test method?
Users must establish appropriate safety and health practices and determine applicable regulatory limitations before use. Specific hazard statements are detailed in Annex A3 of the standard.