Cryogenic Nitrogen Rejection Schemes: Analysis of Their Tolerance to CO2

Abstract

In the “energy transition era” we are experiencing today, natural gas grows strongly and much faster than either oil or coal, being an environmentally friendly fuel supported by a broad-based demand and the continuing expansion of liquefied natural gas (LNG). Natural gas mainly consists of methane, but also some contaminants are present in it. Among them, nitrogen is an inert gas whose content, if too high, must be reduced to levels acceptable for producing a pipeline-quality gas or LNG. When dilution of the high-nitrogen gas stream with a low-nitrogen gas is not practical, a nitrogen removal unit (known as nitrogen rejection unit or NRU) must be installed. This is becoming more and more important as we shift to lower-quality gas feedstocks. Cryogenic distillation is the only viable option for the removal of N₂ on a large scale and in the case of stringent specifications both in the product stream and in the rejected stream. Typically, the feed gas to the NRU has to contain methane and N₂ and only very low quantities of compounds (such as CO₂) that might freeze at the NRU operating temperatures and cause equipment blockage. Focusing on the production of pipeline-quality natural gas, the aim of this work is to analyze the cryogenic removal of N₂ from natural gas streams that also contain CO₂ because a CO₂-tolerant NRU may lower capital and operating costs reducing the upstream removal of CO₂. Different process configurations (i.e., the single-column, the double-column, and the three-column systems) are investigated to determine the maximum allowable CO₂ content in the feed gas that avoids solidification within the process and permits reaching the desired value of the Wobbe index. Therefore, a range of applicability for each configuration is determined depending on the N₂ and CO₂ contents in the feed gas.