Liquified Natural Gas (LNG) gets plenty of attention as a globally traded energy source but some of its most interesting characteristics have little to do with energy. The mention of cryogenic liquids usually raises at least a small amount of interest in all of us. The urban legends of Walt Disney’s cryogenically suspended remains may have spurred the interest but even without the stories, cryogenic temperatures produce unique and often unexpected effects. This short article will remind the reader of some of the cooler aspects of LNG and other cryogenic liquids.
Absolute Zero
Absolute zero is the coldest point on the thermodynamic temperature scale, meaning essentially that all molecular movement has stopped, and the temperature is deemed to be zero. All absolute temperature scales, for example Kelvin and Rankine, have a common zero and thus comparisons of higher temperatures can be made (i.e. 100 K is twice as warm as 50 K). Helium, the lightest Noble Gas, liquifies at about 4.2K and exhibits additional “superfluid” states at even lower temperatures. Liquid nitrogen, the most common cryogenic gas, liquifies around 77 K, while methane liquifies at about 112 K (-162° C or -260° F). Since LNG is primarily methane, the nominal temperature for LNG is usually expressed as 112 K. By applying the absolute scale, LNG is about 28 times warmer than liquid helium and 1.5 times warmer than liquid nitrogen. Ice forms at 273 K, so ice is more than twice as warm as LNG on an absolute scale. The bottom line from this analysis, LNG is very cold, much more similar to liquid nitrogen than any other substance found commonly in everyday experience.
Material Properties
Many high school students are familiar with liquid nitrogen demonstrations in which the temperature of materials is dramatically lowered to alter their physical properties. Using a frozen banana as a hammer and dramatically changing the tone of a bell are two of the most common. Both demonstrations are relevant in the handling of LNG. Elastomers can freeze and become rigid, offering little sealing protection (recall Richard Feynman’s demonstration in front of congress after the Challenger disaster). Metals are also not immune to problems at very low temperatures, and become brittle, as demonstrated by the change in the bell’s tone. Only by the careful selection of all materials used in LNG apparatus, can LNG be safely and efficiently handled.
Volume and Temperature Measurements
The fundamental questions that need to be answered when buying or selling any commodity are all about accurate measurement. For LNG, the volume of liquid transferred is of primary importance along with the energy density. Flow metering is the primary method to determine volumetric exchanges for oil and natural gas alike, but designing a flow meter for use at 112 K is a daunting task. Although there are a handful of flow measurements in LNG performed, the accepted method to measure the volume of LNG is through various forms of tank gauging. Sealing requirements, embrittlement of electronics, and the survivability of moving parts makes most metering impractical. Temperature measurement can also be a challenge. Temperature is often measured using thermocouples, but in a cryogenic environment the preferential method often requires a resistance temperature detector (RTD). The resistivity of many metals is nearly linear over a wide temperature range, with platinum being one of the best characterized, so this fact is exploited to accomplish temperature measurements across wide ranges of cryogenic temperature.
Measuring the temperature of flowing LNG can be especially problematic in slow moving or small transfer lines. Because LNG is usually very near its boiling point, the introduction of even tiny amounts of heat can create gas pockets which destroy the measurement. Gas pockets traveling along the top of large transfer lines are at a different temperature than the liquid. Probe placement and the insulation around the pipe are critical. Pre-vaporization of liquid in gas analysis (sampling) lines create unacceptable temperature variations and large pulsations that can destroy meaningful analysis. Routine measurements in gases and liquids at or near room temperature can be extremely difficult in LNG, simply because of the cryogenic environment.