<![CDATA[Cryogenic air separation units (ASUs) are the most widely applied technology for producing high-purity oxygen and nitrogen for industrial applications. Conventional double-column ASUs generally employ either turboexpander- or Joule–Thomson (JT) valve-based refrigeration systems, whose refrigeration performance strongly influences product distribution and overall energy consumption. Since the amount of refrigeration generated by a turboexpander depends on the fraction of compressed air directed to the expansion process, optimizing the expanded-air fraction offers a promising approach to improve the energy performance of cryogenic ASUs. This study investigates the effect of expanded-air fraction on the performance of a cryogenic ASU and compares turboexpander- and Joule–Thomson (JT) valve-based refrigeration systems using Aspen HYSYS simulation. The simulation was performed using a feed air flow rate of 12,000 kmol.h⁻¹ at 30°C and atmospheric pressure, while the expanded-air fraction in the turboexpander configuration was varied from 20% to 50% of the total feed airflow. Increasing the expanded-air fraction enhanced refrigeration generation and power recovery, resulting in lower column thermal duties and specific power consumption. Gaseous nitrogen production increased from 2,418 to 5,055 kmol.h⁻¹, while liquid nitrogen production decreased from 7,000 to 4,375 kmol.h⁻¹. Temperature profile analysis of the low-pressure column showed that additional refrigeration lowered the operating temperature and improved vapor–liquid equilibrium, promoting oxygen enrichment in the liquid phase and nitrogen enrichment in the vapor phase. Consequently, oxygen purity increased from 97.4% to 98.0%, while nitrogen and oxygen recoveries remained above 99%. The optimum performance was obtained at 50% expanded air, achieving the lowest specific power consumption of 354.5 kWh.ton⁻¹ GOX. Compared with the JT valve configuration, the turboexpander reduced the specific power consumption by approximately 6.1% owing to simultaneous refrigeration generation and mechanical work recovery. These findings demonstrate that optimizing the expanded-air fraction provides an effective operational strategy for improving both product flexibility and energy efficiency in double-column cryogenic ASUs.]]>
