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The fundamentals of feed gas pretreatment

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LNG Industry,

Al Lanning, Wood, examines the processes currently required and in wide use for the pretreatment of LNG feed gas.

The growth and popularity of LNG as an internationally traded energy commodity has allowed large amounts of stranded and excess associated gas to be monetised outside of traditional natural pipeline networks. Natural gas has become a fuel of choice, as it produces fewer emissions compared to coal and fuel oil. Technological improvements in the liquefaction process have driven better operating efficiencies and improved capital cost as train sizes have increased. While natural gas may be a clean fuel when finally delivered to users, it always starts out from the well as a mixture of liquid hydrocarbons, water, sand and other solids, acid gases, nitrogen and metals. Even the cleanest and driest gas wells will contain unwanted impurities. There are many processes and technologies that have been developed over the last 60 – 70 years to treat natural gas before it is used as a fuel or feedstock.

The pretreatment system for LNG plants has evolved into a reasonably consistent and predictable configuration, using proven equipment and systems. While the type and levels of contaminants may vary somewhat based on geography, type of feed gas and delivery arrangement, all LNG plants need protection against a set of critical impurities. These impurities include: mercury, carbon dioxide, hydrogen sulfide and other sulfur compounds, water, heavy hydrocarbons and oxygen. Avoiding equipment damage, downtime and off-plan production requires the removal of these impurities, all of which are complex in their own right.

Mercury (Hg)

Mercury in its various forms may be present in natural gas streams. The concern with mercury being present in feed gas to LNG plants is the very serious danger that it poses to the brazed aluminium used in the construction of cryogenic heat exchangers. Aluminium is very efficient for cryogenic heat transfer, but it is highly susceptible to damage from mercury corrosion. There are three mechanisms by which mercury can attack aluminium. Amalgamation is the process in which mercury forms a liquid solution with various metals, including aluminium. Amalgam corrosion is the combined action of mercury and moisture on susceptible materials like aluminium. Liquid metal embrittlement produces rapid brittle fracture due to the interaction of mercury and aluminium and can cause rapid cracking and severe damage. The solution is to use mercury guard beds, utilising sulfur impregnated carbon, in all new LNG projects, even if gas analysis does not show any detectable levels of mercury.

Carbon dioxide (CO2)

CO2 is present in almost all natural gas streams. It is removed from produced natural gas using several technologies, including amines, in order to meet heating value and inert specifications for pipeline systems, typically in the 2 – 3% maximum range. CO2 is also reduced to low levels before gas enters cryogenic natural gas liquid (NGL) recovery plants – 0.1 to 1.0%, depending on what the recovery target is (lower levels for higher recovery). For LNG feed gas, CO2 must be reduced to low levels – 50 ppmv or less – prior to liquefaction to eliminate the possibility of solid CO2 forming, which would cause plugging problems in the cryogenic heat exchangers. For LNG feed gas from a pipeline system, the actual levels of CO2 will normally be lower than the pipeline maximum specifications, but it is prudent to design the pretreatment system for the pipeline maximum, since the repercussion of not being able to treat all of the feed gas to the required level would be a serious and economically damaging reduction in plant capacity. For LNG feed gas sourced directly from field production, picking the correct design CO2 concentration can be more difficult, since the CO2 value can vary with the life of the field and also as new wells are drilled and produced. There could also be much higher levels of CO2, which can bring other treating technologies into play, such as membranes, which are economical for bringing high CO2 concentrations down to lower levels, but are suitable to go directly to LNG pretreatment at levels of 50 ppmv. A hybrid system using membranes for a high percentage reduction, followed by an amine system for polishing is sometimes an effective combination.

This is an abridged version of an article that was originally published in the December 2019 issue of LNG Industry. The full version can be read here.

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