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Natural Gas for Export

作者:互联网    浏览:14610    发布时间:2015-10-16 11:17:48

Atpresent, more than 110 LNG facilities operate in the U.S. and perform a variety ofservices, according to the Federal Energy Regulatory Commission. Somefacilities export natural gas from the U.S., some provide natural gassupply to the interstate pipeline system or local distribution companies andsome are used to store natural gas for periods of peak demand. There are alsofacilities that produce LNG for vehicle fuel or for industrial use.

 

Theprimary component of natural gas is methane (CH4). This compound gives naturalgas large combustion energy such as the high hydrogen-to-carbon ratio thatboosts the heating value. However, depending upon where the natural gas isextracted, a number of other compounds will be found in the supply. The tableoutlines the major impurities that may exist in natural gas supplies.

 

Thesecond most common component in most cases is ethane (C2H6), although the gasmay also contain fractions of heavier hydrocarbons including propane (C3H8) andperhaps a small amount of butane (C4H10). Natural gas that contains heavierhydrocarbons is known as “wet gas.” By contrast, “dry gas” is almost puremethane. The schematic outlines the major unit processes for LNG production.Not shown due to space limitations are numerous split feeds, auxiliary heatexchangers and byproduct recovery units with the process. Some of these will bediscussed in parts 2 and 3 of this series, particularly as they relate tocondensate return and wastewater treatment requirements.

 

Thefirst unit process is the acid gas scrubber. Although the table does not listany sulfur-bearing compounds, they can easily exist. Those familiar with theoil and refining industries know that “sour” crude oil contains hydrogensulfide (H2S). H2S, besides being a decided safety hazard, is also corrosive.Scrubbing is typically performed with an aqueous amine solution to removecarbon dioxide and sulfur compounds.

 

Thenext step in the process is dehydration to remove water that would otherwisefreeze in the liquefaction process (or could freeze at processing plants thatprepare methane for injection into pipelines). A common process to remove wateris through molecular sieves with zeolites as the absorption media. The poresize can capture water, which can then be periodically expelled to regeneratethe media.

 

Theheavy hydrocarbon recovery unit (HRU), which utilizes distillation, extractsethane and other higher hydrocarbons. In the past, these compounds often wereconsidered to be nuisances in natural gas and might have been discarded.However, in our increasingly energy-efficient world, industry personnel arerecognizing that these hydrocarbons can have great value.

 

Forexample, consider the primary “impurity” ethane. As opposed to C2H6, ethylene(C2H4) has the double-bonded carbon, which is of importance. The value ofethylene as the raw material for many polymers and plastic products cannot beoveremphasized. For many years, most C2H4 has been obtained during crude oilprocessing. However, C2H6 is becoming an increasingly popular feedstock forethylene production, with a technology known as “oxidative dehydrogenation”gaining interest as an alternative to standard cracking. Heavier hydrocarbonsthat are extracted from natural gas also have value. C3H8, of course, serves asheating fuel for many home owners in rural parts of North America, and it is also used for industrial heating applications.

 

Theremaining steps in LNG preparation are liquefaction and nitrogen stripping toproduce the final material for storage. Cooling for liquefaction may utilizehydrocarbon-cooled heat exchangers along with those cooled by more traditionalcoolants. This system, along with the other unit processes already described,require a number of heat exchangers. Some of these are internal to the processwhere natural gas from a unit operation is routed through a heat exchanger withthe inlet natural gas as the feed.

 

However,a number of steam-fed heat exchangers are also necessary. One example is theamine regeneration system in which the sour amine that has collected carbondioxide and hydrogen sulfide is reconstituted as fresh reagent. Another is theheavy hydrocarbon distillation tower(s), where steam can be the heating source.In addition, these unit processes utilize many pumps, compressors and otherfluid transfer equipment. All of these machines require power for operation,and, when considered as a whole, the power requirements are significant.