A significant commercial application of cryogenics is the liquefaction, transport and storage of natural gas. Liquefied Natural Gas (LNG) is generally 95 percent methane with a few percent ethane and much lower concentrations of propane and butane. LNG liquefies at 111.6 K.Unlike many applications of cryogenics, the motivation for using LNG is not the provision of lower temperatures but rather the very large volume reduction (greater than a factor of 600) between natural gas at atmospheric pressure and temperature and LNG.
Volume reduction allows for efficient transport of large amounts of natural gas. The natural gas industry typically uses LNG for sea transport, with regular shipments occurring between producing countries and consuming countries, for example between the Middle East and Japan or between North Africa and Europe.
Once the LNG arrives at a consuming country it is typically converted to high pressure 300 K gas and distributed via pipeline. In some cases, further shipment within the consuming country is also carried out via LNG. Given the amounts of LNG shipped regularly, this is a major industry with a very large commercial value.
Due to its lower emissions upon combustion, both municipalities and corporate groups are increasingly using natural gas as a fuel for buses and other fleet vehicles. Generally, this is accomplished with room temperature compressed natural gas (CNG) but there are uses of LNG as a fuel in maritime ferries. Figure 1 shows an example of such a system, recently installed in a Polish ferry.
LNG provides a rich field for cryogenic engineering. Given the amount of LNG transported, optimization of liquefaction plants is an ongoing effort, frequently using mixed gas refrigeration (Cold Facts Vol 32 No 1). Conversion of LNG back to 300 K gas is often carried out by heat exchange with ambient air or sea water, but work has been carried out to use the cold LNG to assist in air separation (Cold Facts Vol 31 No 2), production of dry ice and freezing of food, thus improving the overall efficiency of the LNG industry.
The design of safe and efficient storage tanks for LNG is another important topic of research and development. Most LNG tanks are not vacuum insulated due to the high temperatures involved and the generally large size of such tanks. Tanks for ferries and other vehicles provide an exception, however, and are sometimes vacuum insulated. Engineers typically insulate large LNG tanks with foam, perlite or other materials. Significant work has been done over the years to ensure safety in LNG systems. In addition to the obvious flammability hazard, LNG as a multicomponent fluid is susceptible to stratification and rollover in storage tanks. Much work has been done to eliminate or mitigate these risks.
Examples of optimizing the liquefaction of natural gas are given in H.-M. Chang, “A Thermodynamic Review of Cryogenic Refrigeration Cycles for Liquefaction of Natural Gas,” in Cryogenics 72, 2015; and H.-M. Chang et al., “Combined Brayton-JT Cycles with Pure Refrigerants for Natural Gas Liquefaction,” in AIP Conference Proceedings 1474, Advances in Cryogenic Engineering: Transactions of the Cryogenic Engineering Conference-CEC, Volume 57B, ed. J.G. Weisend II et al., 2012. Using LNG for cooling as part of an air separation process is given in Zheng Jieyu et al., “Simulation of a Novel Single-column Cryogenic Air Separation Process Using LNG Cold Energy,” in Physics Procedia 67, ed. H.J.M. ter Brake et al., 2015.
Some safety topics in LNG are covered in J.Q. Shi et al., “Numerical modeling and flow visualization of mixing of stratified layers and rollover in LNG,” in Cryogenics 33, 1993; and N.J. Fulford and M.D. Slatter, “Developments in the safe design of LNG tanks,” in Cryogenics 28, 1988. An example of continuing fundamental studies of LNG is given in D. Chen and Y. Shi, “Two-phase heat transfer and pressure drop of LNG during saturated flow boiling in a horizontal tube,” Cryogenics 58, 2013. The use of LNG as fuel in a ferry is described in M. Chorowski et al., “LNG systems for natural gas propelled ships,” in IOP Conference Series: Materials Science and Engineering Volume 101, Advances in Cryogenic Engineering: Proceedings of the Cryogenic Engineering Conference, 2015. ■