Minimum work associated with separating nitrogen from air: An exergy analysis

Abstract

Background Nitrogen is essential for a variety of industries, including heat treatment, laser cutting, fire protection, and food packaging. Many companies in these industries obtain nitrogen via on-premises air separation processes. The three main processes for separating nitrogen from ambient air are cryogenic distillation, membrane separation, and pressure-swing adsorption (PSA). Improvements to these processes will likely focus on increasing efficiency, resulting in reduced environmental impact owing to less electrical power demand and opportunities for economic incentives. Regardless of the process utilized, a minimum theoretical amount of work input is required to obtain nitrogen gas at different pressures and concentrations compared to ambient conditions. Methods An equation was derived to evaluate the total exergy (including thermo-mechanical and chemical exergy) of product and exhaust mixtures resulting from air separation, indicating the minimum theoretical work input as a function of the product pressure, purity, and process recovery rate. This analysis considered an air separation system as a black box, with the input, output, and exhaust assumed to be ideal gas mixtures of nitrogen and oxygen at 15°C. The analysis applies to cryogenic distillation if the product and exhaust mixtures return to the gas phase. Results In general, the minimum required work input increases with product purity and recovery rate. Plots of minimum theoretical work versus product purity and recovery rate were made for two product pressures (atmospheric and 800 kPa) to show the behavior of the derived equation. Conclusions The analysis allows for direct efficiency (based on the second law of thermodynamics) comparisons between existing processes and future technological innovations in the field of air separation. Actual air separation systems have low efficiencies compared to ideal systems; actual PSA systems were estimated to have second law efficiencies of 5.5–11.2%. Therefore, there is great potential for improvements to current air separation systems.