2025 Conference Workshop
Aromatic compounds, such as benzene, toluene, ethylbenzene, and xylene (BTEX), have relatively high frost points in methane, leading to early freezing in the cold section of an LNG plant. Traditionally, these hydrocarbons have been removed in a Scrub Column or in a NGL Recovery unit with a turbo-expander process. Increasingly, LNG plants, especially in North America, are fed by natural gas from the pipeline grid. NGL’s have been extracted from this gas, resulting in a lean gas with a tail of heavy hydrocarbons in trace concentrations.
Liquefied natural gas (LNG) is produced by cooling natural gas to negative 160°C. Prior to cooling to these low temperatures, impurities must be removed from the gas to ensure proper performance of the downstream liquefaction process. A standard pre-treatment line-up consists of an acid gas removal unit (AGRU), a molecular sieve dehydration unit to remove water to <0.1ppm, and a mercury removal unit. It wasn’t until the first baseload LNG plants in the United States processing lean gas started up in the mid-2010s and began to experience freezing in the cryogenic heat exchangers that this typical pre-treatment approach was questioned. Meanwhile, the industry has acknowledged and started to address the freezing problem, which reduces LNG throughput throughout the region.
The freezing of coldboxes in US LNG plants is due to traces of heavy hydrocarbons (HHCs) in otherwise lean natural gas, which are not removed prior to the cold section of the plant. Depending on the design of the plant, HHC freezing can occur as far upstream as the gas/gas heat exchanger upstream of the turboexpander. However, it is more common for the freezing to occur in the cryogenic heat exchanger of the plant. US LNG plants have addressed the HHC freezing problem by reducing throughput or completely shutting down the trains to warm up the coldbox to derime. This process leads to flaring of natural gas and excess energy consumption due to warming and cooling of the coldbox.
It has been shown that for an existing plant and for new LNG development projects adsorptive removal of heavy hydrocarbons and water in a TSA system is a viable option to prevent HHC freezing. Especially as a drop-in solution, implementing adsorptive HHC removal in the pre-treatment section is a powerful approach to debottleneck LNG plant without or with minimal CAPEX spending.
The interactive workshop will address technology fundamentals as well as limitations and advantages of different approaches to address HHC freezeout. Operational data from existing plants will be discussed.
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