(234c) A Comparison of Cryogenic Air Separation Processes Based on Exergy Analysis

Cryogenic distillation is the most widely used technique for industrial-scale production of oxygen and nitrogen from air. A two-column process is commonly employed. This work aims to compare seven (7) alternative two-column designs for the cryogenic air separation process based on exergy analysis.

Steady-state simulation models are developed using Aspen Plus®. The feed conditions (500 ton/h of ambient air with 50% relative humidity) and product purities (99% for both oxygen and nitrogen) are fixed for all models. Steady-state simulation results are used to calculate the exergy efficiencies of various equipment and their contribution to the total exergy destruction. It is observed that the compression section is the largest source of exergy destruction and accounts for ca. 30–45% of the total exergy destruction in the process. The low-pressure column (ca. 20–40%) and the multistream heat exchanger (ca. 10–20%) are the 2nd and the 3rd largest sources of exergy destruction, respectively. Less than 10% of the total exergy destruction occurs in the high-pressure column.

The total exergy destruction rate is ca. 32.6–38.4 MW for various process designs. A thermally-integrated Petlyuk-like arrangement of high- and low-pressure columns, where the air feed is split into 2 fractions, both of which are fed to the high-pressure column, and where both products are obtained from the low-pressure column, offers the lowest total exergy destruction.