CONSTITUENTS THAT AFFECT LNG PROCESSING
Because carbon dioxide freezes into dry ice at these very low temperatures, any CO2 that is present is likely to clog valves and equipment, reducing or preventing gas liquefaction.
There are two primary ways to remove carbon dioxide from natural gas. The first and most common approach involves an amine system, a closed-loop liquid solvent system that absorbs carbon dioxide from the natural gas stream. These systems, which regenerate and reuse amine over and over again, can accommodate a wide range of carbon dioxide concentrations. However, the higher the concentration, the higher the amine circulation rate required. The gas exiting the amine system is water‑saturated and requires dehydration afterwards. The carbon dioxide removed from the feed gas is vented directly to atmosphere or if required, such as when hydrogen sulfide is present, burned in a thermal oxidizer.
In smaller facilities with lower carbon dioxide concentrations, a multiple-bed, molecular sieve (mol‑sieve) system can be used to remove carbon dioxide and water from natural gas at the same time. These systems involve filling beds with adsorbent materials and regenerating them with dry feed gas. They can typically handle concentrations up to 2% (by volume) of carbon dioxide in the feed gas.
Because used regeneration gas in mol-sieve systems contains carbon dioxide that must be purged from the system, the gas must be disposed of by burning it for fuel, flaring it, or returning it to a pipeline. If returned to a pipeline, any carbon dioxide in the tail gas must be monitored so that it does not exceed pipeline limits. Regeneration gas from mol-sieve units cannot be recycled to the front of the plant because that will cause a buildup of carbon dioxide.
While amine systems are more complex than mol‑sieve units, they are typically a better choice for larger LNG systems and those with higher carbon dioxide concentrations because they do not require the disposal of large regeneration gas flows. With both of these systems, carbon dioxide is typically removed down to less than 50 parts per million volume (ppmv).