Abstract
<jats:p>This paper presents the process design, steady-state simulation, and mechanical sizing of a cryogenic air separation unit (ASU) developed as part of a sustainable ammonia production flowsheet from biogas. The unit uses Linde double-column cryogenic distillation to separate atmospheric air (modeled as a ternary O2/N2/Ar mixture) into a high-purity nitrogen stream for ammonia synthesis and an oxygen stream for autothermal reforming of biogas-derived methane. The process was simulated in Aspen HYSYS at a feed rate of 1300 kmol/h, achieving a nitrogen product purity of 99.883% and an oxygen product purity of 95.04%. Detailed plate hydraulic design (Souders-Brown flooding, weeping, entrainment, and downcomer back-up checks) and AIChE tray-efficiency calculations were carried out for both the high-pressure column (HPC) and low-pressure column (LPC), followed by mechanical design of the pressure vessels and an equipment cost estimate. The low-pressure column required 30 theoretical trays (41 actual stages) at a diameter of 1.746 m, with an estimated 2024-indexed cost of $380,368. A reflux-ratio/number-of-stages optimization was performed for the high-pressure column to identify the trade-off between operating and fixed costs. The results demonstrate a technically sound and economically quantified ASU design suitable for integration into a green ammonia production route.</jats:p>