Methane, the simplest hydrocarbon with the chemical formula CH₄, is the primary component of natural gas. This colorless, odorless compound exists as a gas under normal atmospheric conditions. For liquid methane to freeze, the temperature must drop to an extremely cold point, specifically -182.5 degrees Celsius (-296.5 degrees Fahrenheit), at standard atmospheric pressure.
Defining the Freezing and Boiling Points
The temperature at which liquid methane solidifies, or freezes, is very low, making it a cryogenic substance. This freezing point is precisely 90.67 Kelvin (K), which translates to -182.48 degrees Celsius (°C) and -296.46 degrees Fahrenheit (°F). This temperature is technically known as the triple point of methane, where solid, liquid, and gas phases can coexist in equilibrium under a specific, very low pressure.
The opposite phase change, where liquid methane turns back into a gas, occurs at its boiling point of approximately 111.66 Kelvin (K). This corresponds to -161.49 degrees Celsius (°C) or -258.68 degrees Fahrenheit (°F) at standard atmospheric pressure. The extreme difference between these temperatures and typical Earth temperatures highlights the challenge in handling liquid methane.
For methane, the triple point occurs at a very low pressure of only 0.117 bar, which is significantly less than the pressure at sea level on Earth. For general purposes, however, the freezing point and the triple point temperature are nearly identical.
The Physics of Methane Liquefaction
Converting methane from a gas to a liquid is a process of extreme refrigeration, falling into the category of cryogenics, which deals with temperatures below -150°C. This transformation requires overcoming the weak forces holding the non-polar methane molecules together by significantly lowering their energy. To achieve liquefaction, the gas must first be cooled below its critical temperature.
Methane’s critical temperature is approximately -82.6 °C (-116.7 °F), and its critical pressure is 46.0 bar. The critical point is the temperature above which the gas cannot be liquefied, no matter how much pressure is applied. Therefore, the gas must be chilled below this threshold before pressure can be used to condense it into a liquid.
Once the temperature is below the critical point, liquefaction involves a refrigeration cycle that expands and compresses the gas repeatedly, lowering the temperature in stages. For storage and transportation, liquid methane is maintained at its boiling point of about -162 °C at near-atmospheric pressure. This cryogenic state minimizes the energy needed to keep the methane liquid, allowing for efficient handling.
The primary physical benefit of this transformation is a massive reduction in volume. When cooled to a liquid, methane occupies only about 1/600th of its gaseous volume at standard conditions. This density increase is the fundamental reason why methane liquefaction is a necessary and energy-intensive industrial process.
Practical Uses of Cryogenic Methane
The practice of liquefying methane into Liquefied Natural Gas (LNG) is primarily driven by the need for efficient storage and long-distance transport. Since methane gas occupies an enormous volume, shipping it across oceans or storing it in large quantities becomes impractical.
LNG is transported in specialized, heavily insulated tanker ships to terminals worldwide, where it is regasified and distributed through pipelines for use in heating and electricity generation. This ability to move large amounts of energy independent of pipelines has established a global market for natural gas. The process allows countries lacking domestic reserves to import fuel safely and economically.
Cryogenic methane also plays a role in space exploration as a rocket propellant. Methane, often combined with liquid oxygen (LOX), is an attractive fuel choice for reusable launch systems due to its high performance and relatively low cost. The combination of liquid methane and liquid oxygen provides a powerful, clean-burning, and dense fuel source.
Furthermore, methane’s cryogenic properties are relevant to the study of other celestial bodies, such as Saturn’s moon Titan. The average surface temperature of Titan is around 94 Kelvin, which is slightly above methane’s triple point. This temperature permits methane to exist as a liquid, forming lakes and rivers on the moon’s surface, a phenomenon unique in the solar system outside of Earth.