Liquefied Petroleum Gas (LPG) Composition Analysis: Testing Items, Methods, and Industry Application

Liquefied petroleum gas (LPG) composition analysis is a critical laboratory testing service that determines the hydrocarbon profile, sulfur compounds, moisture, and physical properties of propane, butane, and mixed LPG products. Accurate compositional analysis ensures product quality, regulatory compliance, and safe handling across refining, distribution, and end-use applications. This technical article explains the key testing parameters, international standards such as ASTM D2163 and GB 11174, analytical methods, and frequently asked questions related to liquefied gas testing.

Technical Overview

Liquefied petroleum gas (LPG) is a flammable hydrocarbon mixture composed predominantly of propane (C3H8) and butane (C4H10), along with varying amounts of propylene, butenes, and lighter or heavier hydrocarbons. It is produced as a byproduct of crude oil refining and natural gas processing, and is stored and transported under moderate pressure in the liquid state. Because LPG composition varies significantly depending on the source, season, and blending strategy, systematic composition analysis is essential for verifying product specifications and calculating physical properties such as vapor pressure, calorific value, and density.

Composition analysis of liquefied gas serves three primary purposes: confirming conformance with national and international specifications, controlling safety-related impurities such as hydrogen sulfide and total sulfur, and supporting commercial settlement based on energy content. Specifications including ASTM D1835, GB 11174, ISO 8819, and EN 589 define the acceptable limits for hydrocarbon composition, sulfur, moisture, corrosiveness, and evaporation residue. Accredited third-party laboratories routinely perform these analyses for refineries, importers, cylinder-filling plants, and industrial consumers.

Testing Samples

Commercial Propane

Commercial-grade propane, with a minimum propane content typically specified at 90% or higher, is widely used in residential heating, agriculture, and forklift fuel. Analysis verifies propane purity, allowable butane content, propylene limits, and residual methane and ethane, ensuring the product meets the volatility and combustion requirements of the designated specification grade.

Commercial Butane

Commercial butane consists mainly of n-butane and isobutane and is used in blending, aerosol propellants, and portable fuel applications in warmer climates. Testing confirms the combined butane fraction, limits on pentane and heavier hydrocarbons, and the concentration of unsaturated compounds that affect combustion stability and gum formation.

Mixed LPG (Propane-Butane Blends)

Mixed LPG, the most common product in general distribution networks, requires precise determination of the propane-to-butane ratio, which directly governs vapor pressure and cold-weather performance. Seasonal blend specifications differ between summer and winter grades, making compositional verification essential for suppliers and cylinder-filling operators.

Automotive LPG (Autogas)

Autogas used in spark-ignition and dedicated LPG vehicles must comply with stricter limits on propylene, dienes (particularly 1,3-butadiene), sulfur, and volatile residues to protect fuel injection systems, catalysts, and evaporative emission controls. Composition analysis supports compliance with motor fuel specifications and engine manufacturer warranties.

Liquefied Natural Gas (LNG) and Other Liquefied Gases

For completeness, liquefied natural gas (predominantly methane) and specialty liquefied gases such as dimethyl ether (DME) can be analyzed using the same chromatographic framework, with methods adapted for cryogenic or high-volatility sampling. Methane, nitrogen, and heavier hydrocarbon quantification is critical for LNG custody transfer and calorific value determination.

Testing Items

Typical analytical parameters for LPG composition analysis include the following:

• Hydrocarbon composition: methane, ethane, ethylene, propane, propylene, propane dienes, n-butane, isobutane, butenes, 1,3-butadiene, pentanes, and C6+ heavies, reported in mol% or mass%
• Total propylene and unsaturated hydrocarbons content
• Vapor pressure at 37.8 °C (100 °F), calculated from composition or measured directly
• Evaporation residue (mg/100 mL) and oil-soluble residues indicating contamination
• Total sulfur content (mg/kg), including H2S, carbonyl sulfide, and mercaptan sulfur
• Hydrogen sulfide detection by lead acetate method
• Moisture and free water content, including methanol co-solvent screening where applicable
• Copper strip corrosion rating (1A–4A scale) to assess corrosive sulfur species
• Density at 15 °C or 20 °C (kg/m³), calculated or measured
• Lower and higher heating values (MJ/kg) derived from compositional data
• Odorant (ethyl mercaptan or similar) concentration for leak-detection compliance
• Octane number estimation for autogas applications

Testing Methods

Gas chromatography (GC) is the core technique for liquefied gas composition analysis. A liquid sample is withdrawn from the pressurized cylinder or pipeline through a sampling valve, vaporized under controlled conditions, and separated on packed or capillary columns. Key standardized methods include:

• ASTM D2163 – Determination of hydrocarbons in liquefied petroleum gases and propylene concentrates by gas chromatography
• ASTM D1835 – Standard specification for liquefied petroleum (LP) gases, defining grade limits for commercial propane, commercial butane, and propane-butane mixtures
• ASTM D2598 – Calculation of physical constants (vapor pressure, relative density, and motor octane number) of LPG from compositional analysis
• ASTM D1267 – Measurement of gauge vapor pressure of LPG (LPG-Reid method)
• ASTM D1837 – Volatility of liquefied petroleum gases (evaporation residue assessment)
• ASTM D2784 – Sulfur in liquefied petroleum gases by oxy-hydrogen burner or lamp method; trace sulfur is also measured by UV fluorescence per ASTM D6667 or related techniques
• ISO 8819 – LPG detection of hydrogen sulfide using the lead acetate method
• ISO 4256 – LPG determination of gauge vapor pressure
• GB 11174 – Chinese national standard for liquefied petroleum gas, specifying composition, total sulfur (≤ 343 mg/m³), H2S (pass the copper strip corrosion test), and evaporation residue limits
• GB/T 10410 and related chromatographic procedures for gas analysis in Chinese regulatory practice

Detectors commonly employed include the thermal conductivity detector (TCD) for permanent gases and light hydrocarbons, and the flame ionization detector (FID) for hydrocarbon quantification at high sensitivity. Gas chromatography–mass spectrometry (GC-MS) is used for confirmatory identification of trace components. Moisture is determined by Karl Fischer titration or electronic moisture analyzers, while copper strip corrosion testing per ASTM D1838 evaluates the aggressive sulfur compounds that could damage cylinders, valves, and copper-bearing equipment. Reported results are typically accompanied by calculated properties such as vapor pressure, density, and calorific value derived from the molar composition using standard equations of state.

Applications

Composition analysis of liquefied gas supports quality and safety decision-making across the entire value chain:

• Refinery and gas plant quality control: verifying stream composition before storage, blending, or shipment
• Import and export inspection: demonstrating conformity with contractual specifications and destination-country regulations for customs clearance
• Cylinder filling and distribution safety: confirming vapor pressure limits and excluding corrosive sulfur compounds that compromise cylinder integrity
• Commercial settlement and custody transfer: calculating energy content and mass-volume conversion for billing accuracy
• Automotive fuel certification: ensuring autogas meets propylene, diene, and sulfur limits for engine durability and emissions compliance
• Incident and complaint investigation: identifying contamination, off-specification blending, or adulteration following equipment damage or consumer complaints
• Process optimization: monitoring depropanizer, debutanizer, and alkylation unit performance through stream composition trending
• Research and certification: supporting new product registration, MSDS development, and hazardous chemicals classification under regulations such as GHS

FAQ

What is the difference between LPG and LNG composition analysis? LPG analysis focuses on C3 and C4 hydrocarbons (propane, propylene, butanes, butenes) along with sulfur and residue limits, while LNG analysis is dominated by methane with quantification of nitrogen, ethane, and trace heavier components for calorific value and custody transfer. Both rely on gas chromatography but differ in sampling hardware and calibration gases.

How is an LPG sample collected safely? LPG must be sampled in the liquid phase using pressurized stainless-steel cylinders or floating-piston cylinders equipped with excess-flow valves. Sampling is performed by trained personnel following the sampling procedures referenced in ASTM D1265 or equivalent standards, with vaporization occurring only at the analytical instrument inlet.

Why is propylene content limited in LPG specifications? Propylene and other olefins burn hotter than paraffinic propane and butane, which can stress burners and engine components, and they contribute to gum and deposit formation. Commercial propane specifications typically cap propylene content to ensure clean, stable combustion.

What does a failed copper strip corrosion test indicate? A rating worse than the specification limit (commonly 1A or 1B) indicates the presence of corrosive sulfur species such as elemental sulfur or hydrogen sulfide, which can attack copper and brass components in valves, regulators, and appliances, signaling a safety and equipment-durability risk.

How long does composition analysis take? Routine single-sample chromatographic analysis is completed within a few hours of instrument availability; full specification testing, including sulfur, moisture, residue, and corrosion testing, is typically reported within 3 to 7 working days depending on the testing scope and laboratory workload.

Which standards should a Chinese-market LPG product comply with? Products sold in China are generally evaluated against GB 11174 for commercial LPG, covering composition, vapor pressure, evaporation residue, total sulfur, copper strip corrosion, and H2S criteria, supplemented by international specifications such as ASTM D1835 when export contracts require them.