1. Field
The present disclosure relates to liquefied natural gas, and more particularly, to a method for transporting and processing liquefied natural gas with butane component.
2. Discussion of the Related Technology
In recent, the amount of consumption of natural gas has been increased rapidly throughout the world. The gas well at which natural gas is produced is generally far away form consumers at which natural gas is consumed. Accordingly, natural gas which is in a gas state is transported through a gas pipe line installed on the land or in the sea, or natural gas which is in a liquefied natural gas (LNG) state is transported by an LNG carrier (LNG transport vessel) to distant consumers while the liquefied natural gas is stored in the LNG carrier. Liquefied natural gas is produced by cooling natural gas at an extremely low temperature of approximately −163° C., and a volume of the liquefied natural gas is approximately 1/600 of a volume of natural gas which is in a gas state, so that marine transportation is suitable for a long-distance transportation of liquefied natural gas.
Immediately after natural gas is produced from a gas well, impurities such as moisture, carbon dioxide and the like as well as various hydrocarbon components are contained in the natural gas. Such impurities may cause problems in that when natural gas is cooled at extremely low temperature, the impurities are cooled to clog pipe lines of facilities for forming or treating liquefied natural gas.
Before natural gas is liquefied, accordingly, all components capable of being cooled should be removed from the natural gas. In general, moisture, carbon dioxide and heavier hydrocarbon components comprising 5 or more carbon atoms (C5+) contained in natural gas are separated from natural gas before the liquefaction of the natural gas.
Generally, hydrocarbon components except methane, i.e., ethane, propane and butane (C2˜C4), are separated during the liquefaction of natural gas, and the natural gas is divided into the various hydrocarbon components. Then, the respective components are separately stored and transported to supply them to consumers. Gas produced by liquefying gas having separated propane and butane as a main component is called as liquefied petroleum gas (hereinafter, referred to as “LPG”) and is widely used as industrial or household fuel.
Various methods for liquefying natural gas, including the step of separating natural gas into hydrocarbon components except methane at a low temperature, have been disclosed. These methods of liquefying natural gas are disclosed in U.S. Pat. Nos. 3,763,658, 4,065,278 and 5,325,673, European Patent No. 0,535,752, WO 90/00589, and the like. In addition, WO 2002/32810 and Japanese Patent Laid Open Publication No. 10-28837 disclose a pretreatment process for removing various impurities before liquefying natural gas.
On the other hand, LNG transported to a consumer is re-gasified and then is supplied in a gas state to consumers. At this time, there is need to adjust a heating value of natural gas, which is re-gasified and supplied to the consumer, required according to a Wobbe Index. The Wobbe Index (WI), which is an index indicating the magnitude of input heat energy with respect to a combustor, is represented by a function of heating value and specific gravity and used as a measure for determining the gas compatibility. If a heating value of the transported natural gas is higher than a standard heating value required by a consumer, the proper amount of nitrogen is mixed to the natural gas.
The foregoing discussion in this section is to provide general background information, and does not constitute an admission of prior art.