Several methods of liquefying a natural gas stream thereby obtaining liquefied natural gas (LNG) are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and does not need to be stored at a high pressure.
The costs in creating and running a liquefying natural gas (LNG) plant or system are naturally high, and much is for the cooling configurations. Thus any reduction in the energy requirements of the plant or system has significant cost benefit. Reducing any cost of any cooling configuration is particularly advantageous.
U.S. Pat. No. 6,272,882 B1 relates to a process of liquefying a gaseous, methane-enriched feed to obtain a liquefied product. The liquefaction process comprises a number of steps, one of which is to separate the partly-condensed refrigerant for the main heat exchanger into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction. At least part of the liquid refrigerant fraction is cooled, liquefied and sub-cooled against off-gas removed from a flash vessel used after the main heat exchanger. The process of U.S. Pat. No. 6,272,882 B1 shows a single ‘train’ for liquefaction.
U.S. Pat. No. 6,389,844 B1 relates to a plant for liquefying natural gas. More specifically, it discloses a pre-cooled dual heat exchanger, dual refrigerant system. The plant in U.S. Pat. No. 6,389,844 B1 has a liquefaction capacity that is 40 to 60% higher than that of a single liquefaction train, and comprises one pre-cooling heat exchanger, and at least two main heat exchangers. Each main heat exchanger uses a main refrigerant, which is separated into a heavy liquid fraction and a light gaseous fraction which are only seen to be cooled in the main heat exchanger, prior to expansion.
It is an object of the present invention to improve the efficiency of a liquefying plant or method.
It is a further object of the present invention to reduce the energy requirements of a liquefying plant or method.
It is another object of the present invention to provide an alternative method and apparatus for liquefying a hydrocarbon stream.