Moisture Measurement and Liquified Natural Gas

Protecting supply lines, from liquification to regasification

Liquified Natural Gas (LNG) is an increasingly important global source of energy.  The market analysts Research and Markets predict that annual demand will grow by almost 9 % CAGR to 2026. 

These figures are echoed by the latest Shell LNG market outlook, which highlights the future role that LNG is likely to play in decarbonization, by acting as both a cleaner alternative to coal-fired power and a back-up supply for renewable power sources.  The Shell report does, however, warn that in the longer term there may be a significant discrepancy between demand and supply, potentially reaching between 200 and 300 million tonnes per annum (MTPA) by 2040. 

In the near term, the growth in demand, combined with market and cost volatility, is placing even greater pressure on existing natural gas production and distribution networks, in terms of maintaining system reliability and gas quality, while protecting operating margins. 

The liquification and regasification of natural gas

Although experiments to liquify gases have been conducted since the 18th century, it was not until the late 19th century that scientists were successful in liquifying methane, the main constituent of natural gas.  It took until the 1940s for the first commercial liquification plant to be opened in the USA, with initial transportation of LNG taking place in 1959 when the purpose-built ship, the Methane Pioneer, sailed between Louisiana and Canvey Island in the UK. 

Today, liquification and regasification of natural gas has become an integral and vital part of our global energy supply, allowing large volumes of gas to be transported safely and profitably from often remote locations to each point of use.   

The process begins by passing natural gas through a series of parallel pipes and vessels – known as ‘trains’ – where the heavier liquids and impurities are removed.  Other impurities, including carbon dioxide and hydrogen sulphide, are then removed using water-based solvents, with any lighter Natural Gas Liquids (NGL) such as propane and butane being extracted for use separately, either as a commercial by-product or as a refrigerant later in the cooling process. 

The resulting gas, primarily methane with a small percentage of ethane, is then ready for liquification.  This takes place in heat exchangers where it is cooled to -162 °C and the volume is reduced by a factor of 600. The resulting clear, colorless and non-toxic liquid is ideal for storage and transportation over long distances.  On arrival, it is then heated to return it to a gaseous state, suitable for burning in power generation systems. 

Trace moisture in LNG 

The process described above removes most of the water found in natural gas.  There is, however, always the risk that trace levels of vaporized moisture will remain in the gas.  This can condense and freeze into ice crystals.  Under pressure, moisture can also create lattice-like structures around methane molecules, which subsequently form as solid hydrates.  In each case, these contaminants can cause blockages in pipes or valves.   

There is also the potential for moisture to combine with other contaminants such as hydrogen sulphide or carbon dioxide to form corrosive acids, which will attack metal surfaces.  Additionally, after regasification there is the risk that moisture in a gaseous stage may condense, causing corrosion of pipework.  Finally, although rare, a phenomenon known as ‘rapid phase transition’ can occur, where LNG that comes into contact with water rapidly expands and explosively releases high levels of energy.  

The importance of trace moisture measurement 

Preventing these problems requires strict control of all process conditions, from extraction of the natural gas to its ultimate point of combustion.  In particular, precise monitoring of moisture is critical, as even extremely low or trace levels of moisture will cause problems.  

Moisture is normally removed from natural gas using molecular sieve dehydration systems, with the gas being forced through a bed of a desiccant such as zeolite (an aluminosilicate mineral) held in a binder material, normally clay.  Zeolite has an open cage-like structure with multiple channels, making it a highly effective material for absorbing water molecules. 

Most systems will use differently sized particles of zeolite, which are arranged in layers within multiple dehydration columns.  The columns function simultaneously, with individual columns being regenerated in sequence by back-flushing dry gas at high temperatures across the desiccant material.  This process does, however, require considerable levels of energy and can be time consuming, with each regeneration cycle requiring periods of heating, cooling and stand-by before the column can be brought back on-line. 

The key to efficient operation is precise monitoring of trace moisture levels of gas as it leaves the dehydration system.  Armed with data from, for example, one of our latest Quartz Crystal Microbalance analyzers, it becomes possible to ensure gas quality, protecting downstream systems and meeting the required commercial and technical specifications, while optimizing the operation of each dehydration column.  

In particular, moisture measurement allows the frequency of regeneration cycles to be decreased, enabling each column to remain on-line for longer without affecting gas quality and offering considerable energy savings.  Precise trace moisture monitoring also allows detailed performance profiles to be created for each dryer column, giving plant operators the confidence to extend the functional life of desiccant materials, often well beyond scheduled replacement dates.  This has a direct benefit in terms of plant availability and operating costs. 

Trace moisture measurement systems

Our systems are custom engineered and include sophisticated analysis instruments, based on fast-response QCM (Quartz Crystal Microbalance) technology, plus all ancillary equipment and fittings.  These systems are capable of providing near real-time measurement of ultra-low trace moisture content down to 0.01 ppmV. 

Click here to learn more about our solutions for natural gas processing.

At Michell Instruments, we have many years of experience designing and manufacturing advanced in-line trace moisture measurement systems, for use in LNG molecular sieve dehydration and other moisture measurement applications in the natural gas sector. If you would like to discuss your requirements, please contact our team today

Found this article interesting? Check out our blog on Precise Measurement of Moisture in Natural Gas

Sources:

Research and markets LNG market report

Shell LNG market outlook 2022

Science Direct