Proceedings magazine is a communication tool for the Coast Guard's Marine Safety & Security Council. Each quarterly magazine focuses on a specific theme of interest to the marine industry.
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31 Fall 2015 Proceedings www.uscg.mil/proceedings What is Liquefaction? The liquefaction process is complex but can be simplified into three primary stages: • pretreatment, • liquefaction, • storage and loading. Pretreatment Prior to liquefaction, natural gas must be treated to remove all components that will freeze at a liquefaction temperature or may compromise process equipment. These include carbon dioxide, sulfur prod- ucts, water, mercury, and heavier hydro- carbons like benzene. When purifed and liquefied into finished byproducts, the heavy hydrocarbons are referred to as natural gas liquids and include ethane, propane, butane, isobutane, and pentane. This stage may occur either at the source or at the liquefaction facility. If the pretreat- ment units are located at the source, further treatment or fltration is neces- sary at the liquefaction facility before the liquefaction stage. Liquefaction During the liquefaction stage, a gas is sub-cooled and liquefed using a process similar to that used in freezers or air conditioners. Generally, LNG is cooled to -161ºC and ethane to -60ºC when stored at near atmospheric pressures. Liquefed petroleum gases are stored in pressurized containers, and therefore will be in liquid form at ambient temperatures. Several liquefaction technologies have been developed to balance the industry demand for fexibility, facility size, cost, and efciency, including cascade units, dual mixed refrigerant, and single mixed refrigerant units. Cascade processes were traditionally used in the early days of the industry and used pure hydrocarbon refrigerant loops in series to condense gas. However, these processes were costly and complicated to operate. Mixed refrigerant processes, true to their name, incorporate a mixture of hydrocar- bons as opposed to a pure refrigerant for more efcient heat transfer. Dual mixed refrigerant processes require two refrig- erant compression systems to achieve the cooling needed to liquefy the gas and are typically used for large-scale facili- ties. Single mixed refrigerant processes provide liquefaction capabilities with a smaller footprint, simplifed operations and maintenance, and lower overall capital and operating costs. Storage and Loading Once the gas is liquefed, it is stored and loaded for transport. Additionally, as energy demands increase in established and emerging markets, gas resources will continue to dem- onstrate promise as a fuel source. In turn, liquefaction will most likely become a necessary cog in the world energy market, as through this technology, essential fuel commodi- ties will be accessible to regions of the world that would otherwise go without. About the authors: Ms. Jennifer Seitter is a process engineer at Black & Veatch with experience designing natural gas liquefaction facilities and gas processing facilities. Mr. Javid Talib is a vice president at Black & Veatch and is the foating LNG program manager. Bibliography: U.S. Energy Information Administration, Natural Gas Gross Withdrawals and Produc- tion, www.eia.gov, 2014. Federal Energy Regulatory Commission, "LNG," www.ferc.gov. Gas Processing, "US Natural Gas Plant Field Production of NGL, LPG, ethane/ ethylene and propane/propylene, Mbpd," 2014, p. 8. releases are sent to a flare to safely combust. Staff must ensure there are no unintentional leaks, so they place hydro- carbon detectors throughout the facility to monitor for leaks. In addition, as with any type of refning unit, these facili- ties should be designed to ensure safe operation through adequate relief points, proper operation and maintenance procedures, and personnel safety training. Typical safety evaluations include proper response to gas dispersion, fare radiation, blast and over-pressurization, controlled depressurization or blow-down, dropped objects, and spill containment. Looking Ahead Developing increasingly effcient liquefaction technology and its application to onshore, near-shore, or offshore facili- ties will continue to create opportunities to tap gas resources once thought unfeasible, so liquefaction will continue to be a key objective for energy companies. Transportation Refrigeration Liquefaction Storage & Loading Transportation & Marketing Condensate removal CO 2 removal Dehydration Mercury & H 2 S removal Treatment