Liquefied Petroleum Gas (LPG) is a versatile fuel source composed primarily of a flammable mixture of propane (\(\text{C}_3\text{H}_8\)) and butane (\(\text{C}_4\text{H}_{10}\)). This fuel is colorless and naturally odorless, but a chemical odorant, typically a volatile mercaptan, is added for safety to allow leak detection. LPG is easily liquefied under low pressure, which significantly reduces its volume, making it highly efficient for storage and transport. The resulting liquid fuel is widely used for domestic cooking, heating, industrial applications, and as an alternative vehicle fuel (Autogas).
Primary Sources of LPG
The world’s LPG supply originates from two major industrial processes. Approximately 60% of all LPG is recovered during the processing of raw natural gas extracted from the earth. This raw gas, often called “wet” natural gas, contains Natural Gas Liquids (NGLs), including propane, butane, and other heavier hydrocarbons.
The remaining supply, roughly 40%, is produced as a co-product during the refining of crude oil at petroleum refineries. Since LPG is a co-product rather than the main goal, the amount recovered from crude oil refining varies depending on the specific refinery processes and market demand.
Extracting LPG During Natural Gas Processing
The extraction of LPG from natural gas begins at processing plants where the raw gas stream is treated to remove impurities and heavier components. The primary goal is to separate the desired NGLs (propane and butane) from the lighter methane gas, which is the main component of marketed natural gas. This separation prevents heavier hydrocarbons from condensing in pipelines and reducing gas quality.
The separation process often involves cooling the gas stream to extremely low temperatures using cryogenic processing. In this system, the gas is chilled using heat exchangers and turbo-expanders, causing the NGLs to condense into a liquid state while methane remains a gas. This fractional separation leverages the distinct boiling points of the components: propane boils at about -42°C and butane at approximately -0.5°C.
Absorption Process
Another technique used to isolate LPG components is the absorption process, which passes the natural gas through a lean oil or similar solvent. The heavier hydrocarbons are absorbed by the solvent, while the methane passes through. Following separation, the NGL stream is sent to a fractionation unit where individual components, such as ethane, propane, and butane, are distilled and purified into their final commercial grades.
Recovering LPG During Crude Oil Refining
LPG is recovered in petroleum refineries through several distinct processes that separate the complex mix of hydrocarbons found in crude oil.
Atmospheric Distillation
The initial stage is atmospheric distillation, where heated crude oil is vaporized and separated based on boiling points in a distillation column. Propane and butane, having the lowest boiling points, are collected as part of the overhead gases at the top of the column.
Conversion Processes
More substantial recovery occurs during conversion processes designed to produce lighter, higher-value products like gasoline. Processes such as catalytic cracking and hydrocracking intentionally break large molecules of heavier oil fractions into smaller ones. This molecular breakdown generates a significant amount of propane and butane as a byproduct.
The gaseous mixture containing the LPG components must then undergo further purification, known as sweetening, to remove undesirable compounds like hydrogen sulfide (\(\text{H}_2\text{S}\)). Specialized columns, such as debutanizers and deethanizers, are used to further separate and fractionate the gases into commercial-grade propane and butane. The yield of LPG from crude oil refining typically represents a small percentage, often between 1% and 4% of the crude oil processed.
Storage, Transportation, and Safety
LPG’s ability to be liquefied reduces the gas volume by approximately 250 times, allowing large quantities to be stored and shipped efficiently. LPG is typically stored in robust, pressurized steel vessels, ranging from small domestic cylinders to large bulk storage tanks and seagoing tankers.
For large-scale transport, LPG is moved via dedicated pipelines, specialized rail cars, and massive marine tankers designed to handle the pressurized liquid. The pressure required to maintain the liquid state varies with composition and temperature; for instance, pure propane requires a higher pressure than pure butane. Storage tanks are never filled completely, typically to 80–85% of capacity, to allow for the liquid’s thermal expansion as temperatures fluctuate.
Handling this pressurized, flammable product requires strict safety protocols. Since LPG vapor is significantly denser than air, any leak will cause the gas to settle in low-lying, confined areas, creating a fire or explosion hazard. Therefore, storage areas must be well-ventilated to prevent dangerous gas accumulation. Cylinders must always be stored upright and secured to prevent damage to the valves or accidental release.