Propane is a simple hydrocarbon molecule, commonly known as Liquefied Petroleum Gas (LPG). It is a naturally occurring component of raw fossil fuel streams, recovered and refined as a byproduct during the processing of two primary energy sources: natural gas and crude oil. Because propane is a naturally lighter and more volatile component, specialized industrial processes are required to separate it from the bulk raw material. The journey from an underground reservoir to a usable fuel tank involves distinct extraction, recovery, and final preparation stages.
Extraction During Natural Gas Processing
Raw natural gas contains mostly methane, but also holds a mixture of heavier hydrocarbons known as Natural Gas Liquids (NGLs), which include ethane, butane, and propane. Propane must be separated from the primary methane stream because it would condense into a liquid inside pipelines, causing clogs and disrupting the flow of natural gas. Specialized plants use physical separation techniques to isolate propane and the other NGLs.
The most efficient method for extraction is the cryogenic expansion process, which utilizes extreme cold to separate the components. The raw gas stream is chilled to very low temperatures, sometimes reaching around -120 degrees Fahrenheit. This rapid cooling causes the heavier NGLs, including propane, to condense into a liquid form while the lighter methane remains a gas.
An alternative process is absorption, where the raw gas is passed through an absorption tower containing a chilled, low-volatility oil. The heavier NGL components are selectively absorbed by this oil, while the methane gas passes through. The resulting “rich” oil is then heated to boil off the absorbed NGLs, which are then collected and sent for further fractionation. Both methods ensure that propane is successfully removed from the natural gas stream for commercial use.
Recovery During Crude Oil Refining
The second source of propane is the refining of crude oil. The initial separation occurs through fractional distillation, where the heated crude is vaporized and allowed to rise through a tall distillation column. Hydrocarbons condense at different points in the column based on their unique boiling points.
As a very light hydrocarbon, propane is one of the fractions that condenses at the lowest temperature, emerging near the top of the column along with butane and other refinery gases. This initial separation provides a portion of the world’s propane supply.
Additional propane is generated through a process called “cracking.” Refineries use cracking to break down larger, heavier, and less valuable hydrocarbon molecules into smaller, more marketable ones like propane and gasoline. This conversion process is necessary because market demand for lighter fuels often exceeds the amount produced through simple distillation.
Purification and Odorization
After its initial recovery, the raw propane stream must be purified to meet commercial standards. This stage involves removing impurities, such as trace sulfur compounds and water, to ensure the final product is clean-burning. Once purified, the propane is ready for its final preparation step: odorization.
Propane is naturally colorless and odorless, which presents a significant safety hazard in the event of a leak. To make the gas detectable, a warning agent, almost always a chemical called ethyl mercaptan, is injected into the liquid propane. Ethyl mercaptan is a sulfur-containing compound with a distinct, pungent smell often described as rotten cabbage or skunk-like. Finally, the purified and odorized propane is compressed and stored as a liquid under pressure for efficient transport and distribution to consumers.