LPG comes from two main sources: processing raw natural gas and refining crude oil. In both cases, propane and butane (the two primary components of LPG) are separated out from heavier and lighter hydrocarbons through heating, cooling, and distillation. Roughly 60% of the world’s LPG supply comes from natural gas processing, with the remainder produced in oil refineries.
What LPG Actually Contains
LPG is a blend of propane, butane, and smaller amounts of propylene and butylene. The exact ratio depends on the intended use and the climate where it will be burned. In colder regions, LPG leans heavily toward propane because propane vaporizes at lower temperatures, making it more reliable in winter. In warmer climates, butane works fine and is often cheaper to produce. Commercial-grade propane, engine-fuel-grade propane (sometimes called HD-5), and commercial-grade butane are the three standard grades sold as heating and engine fuels.
Extracting LPG From Natural Gas
Raw natural gas pulled from underground wells isn’t pure methane. It contains a mix of heavier hydrocarbons, including ethane, propane, butane, and sometimes pentane. These heavier components need to be removed before the gas can be sent through pipelines, partly because they raise the gas’s energy content above pipeline specifications and partly because they’re more valuable when sold separately.
The separation happens at gas processing plants using a technique called fractionation. The raw gas is cooled to extremely low temperatures, which causes the heavier hydrocarbons to condense into liquids while methane stays as a gas. These condensed liquids, known as natural gas liquids, then pass through a series of distillation columns. Each column is tuned to boil off a specific component based on its boiling point. A column called a demethanizer strips out any remaining methane first. Subsequent columns separate ethane, propane, and butane into individual streams. The propane and butane streams become LPG.
Modern plants take advantage of the extreme cold already present in the process. When liquefied natural gas is regasified (warmed back into a gas for pipeline delivery), the cryogenic temperatures can be used to fractionate out propane and butane with very low energy costs, since the cold does much of the separation work for free.
Producing LPG in Oil Refineries
The second major source of LPG is crude oil refining. When crude oil enters a refinery, it’s heated in a furnace and fed into a tall atmospheric distillation tower. Inside the tower, different components separate by weight. The lightest fractions, including LPG gases, vaporize and rise to the very top, where they cool and condense back into liquid. Heavier products like diesel and fuel oil collect at lower levels.
A standard 42-gallon barrel of crude oil yields about 1.72 gallons of LPG, roughly 3.8% of the barrel’s total output. That’s a small fraction compared to the 19 gallons of gasoline the same barrel produces, but refineries process millions of barrels daily, so the volumes add up quickly.
Refineries also generate additional LPG through cracking, a process where heavier hydrocarbons are broken into lighter ones using high heat and catalysts. Fluid catalytic cracking units, which exist in most complex refineries, produce significant quantities of propane and butane as byproducts while converting heavy oils into gasoline. The LPG from cracking tends to contain more propylene and butylene than the LPG from simple distillation, which matters for chemical manufacturing but less so for heating fuel.
Removing Impurities
Raw LPG straight from a distillation column or cracking unit isn’t ready for your grill or furnace. It contains impurities, primarily hydrogen sulfide, carbon dioxide, and various sulfur compounds called mercaptans. These contaminants are corrosive, smell terrible, and produce harmful emissions when burned. Water is also present and needs to be removed to prevent ice formation in valves and regulators.
The cleanup process is called “sweetening,” an industry term for removing sulfur-based contaminants from “sour” gas. The most common method pumps the raw LPG through a chemical solution containing alkanol amines, compounds that react with and absorb hydrogen sulfide and carbon dioxide. The contaminated solution is then heated in a separate vessel to release the captured gases, regenerating the amine for reuse. For stubborn sulfur compounds, refineries may follow up with a caustic scrub or pass the LPG through molecular sieves, which are porous materials that trap specific molecules based on size. Molecular sieves can simultaneously remove both sulfur compounds and moisture in a single step, though the equipment is more expensive to operate.
Adding the Distinctive Smell
Pure LPG is colorless and odorless, which would make leaks impossible to detect by smell alone. To solve this, producers inject a chemical called ethyl mercaptan before the gas reaches consumers. The standard industry practice adds about 1.0 to 1.5 pounds of ethyl mercaptan per 10,000 gallons of LPG, a concentration set by the National Fire Protection Association. Even at these tiny concentrations (roughly 16 billionths of a gram per milliliter of vapor), ethyl mercaptan produces the strong, unmistakable “rotten egg” smell that alerts you to a gas leak.
Pressurizing, Filling, and Storing
LPG gets its name from the fact that these gases become liquid under moderate pressure at room temperature. Propane liquefies at about 120 psi at 70°F, and butane at even lower pressures. This property makes LPG remarkably efficient to store and transport: the liquid takes up about 270 times less space than the same amount of gas at atmospheric pressure.
At bottling plants, LPG is pumped into steel or composite cylinders using automated filling systems that measure by weight. The filling mechanism shuts off automatically once the cylinder reaches a preset total weight, ensuring each tank holds the correct amount with adequate space left for the liquid to expand as temperatures change. A single operator can typically manage four filling stations simultaneously. Larger quantities move in bulk via pressurized tanker trucks, rail cars, or ocean-going vessels to distribution terminals, where the LPG is stored in pressurized tanks or sometimes in underground caverns carved from salt formations.
Quality Standards for the Final Product
Before LPG reaches consumers, it must meet specifications set by ASTM International (standard D1835 in the United States). These requirements cover vapor pressure, volatile residue, density, and corrosiveness. Vapor pressure limits ensure the gas performs correctly in appliances across a range of temperatures. Residue limits guarantee that virtually all the liquid in the tank will vaporize cleanly, leaving no gummy deposits in burners or engines. Corrosion tests confirm that remaining sulfur levels won’t damage copper fittings and fuel lines. Four basic types of LPG are defined under this standard, covering domestic heating, commercial and industrial use, and engine fuel applications.