CO2-Tolerant Cryogenic Nitrogen Rejection Schemes: Analysis of Their Performances

In the transition to a sustainable energy future, natural gas is a key player supporting a shift away from coal, where renewables and other nonfossil fuels may not be able to grow sufficiently on their own. The growing importance of natural gas has led to a re-evaluation of the potential of unconventional, stranded, and contaminated gas reserves that were previously considered economically unviable. Among them, nitrogen-rich natural gas feedstocks, which in the past were thought to be a not-so-interesting methane source, are now becoming a considerable fraction of the total treated gas. For this kind of low-quality gases, nitrogen removal is necessary to lower the inert content and to produce a pipeline-quality gas or liquefied natural gas (LNG). Considering the available nitrogen removal technologies, cryogenic nitrogen rejection is nowadays the leading one for large-scale applications, with capacities exceeding 0.5 million standard cubic meters per day (MSCMD), being very flexible regarding the inlet N2 content. Depending on the feed composition, different nitrogen rejection units (NRUs—i.e., the single-, the double-, and the three-column systems) are available for treating inlet gas mixtures. The aim of the present work is to evaluate the performances of different cryogenic nitrogen rejection schemes through energy and exergy analysis. Specifically, single-column and three-column nitrogen rejection schemes have been considered with various natural gas feed compositions, focusing on the range where different nitrogen removal schemes are applicable. The net-equivalent methane analysis accounts for the amount of methane required to supply the overall process energy demands through specific processes assumed as reference. On the other hand, exergy analysis evaluates the exergy efficiency of each process scheme through a thermodynamically rigorous approach, converting energy and material flows into their exergy equivalents. Results prove that the three-column process scheme reaches the highest thermodynamic performances, resulting in the best values of exergy efficiency and equivalent methane requirements with respect to the other configurations. This is mainly due to the lower prefractionation column condenser duty, resulting in a less irreversible heat exchanging process.