Comment on Hamayun et al. Evaluation of Two-Column Air Separation Processes Based on Exergy Analysis. Energies 2020, 13, 6361

Abstract

Oxygen production from air belongs to energy-intense processes and, as a result, possibilities for its decrease are a frequent topic of optimization studies, often performed with simulation software such as Aspen Plus or Aspen HYSYS. To obtain veritable results and sound solutions, a suitable calculation method hand in hand with justified assumptions and simplifications should form the base of any such studies. Thus, an analysis of the study by Hamayun et al., Energies 2020, 13, 6361, has been performed, and several weak spots of the study, including oversimplified assumptions, improper selection of a thermodynamic package for simulation and omission of certain technological aspects relevant for energy consumption optimization studies, were identified. For each of the weak spots, a recommendation based on good praxis and relevant scientific literature is provided, and general recommendations are formulated with the hope that this comment will aid all researchers utilizing Aspen Plus and Aspen HYSYS software in their work.

Reply to Variny et al. Comment on “Hamayun et al. Evaluation of Two-Column Air Separation Processes Based on Exergy Analysis. Energies 2020, 13, 6361”

“…The assumption of no pressure drop or fixed pressure drop in heat exchangers is already reported in the literature for air separation processes [3][4][5]. If we used more rigorous equations for pressure drop calculations in heat exchangers, e.g., those proposed by Variny et al [1], the pressure drop would indeed be different for different exchangers in different configurations. This would also give rise to apparent performance differences between different exchangers, their importance being proportional to their magnitudes.…”

“…This means that the adsorbers are similarly sized in all configurations, produce similar pressure drop, and require similar energy for their regeneration. In short, this consideration would change the absolute values of total energy consumption in different configurations, as correctly pointed out by Variny et al [1], but would not alter their relative order.…”

“…This is a reply to the paper by Variny et al [1] who have commented on our recently published work, "Evaluation of Two-Column Air Separation Processes Based on Exergy Analysis" [2].…”

“…We are thankful to Variny et al [1] for highlighting this issue. In configurations C1-C6 as reported in our published work [2], adsorbers precede multi-stream heat exchangers, and it is safe to assume that the air entering the cryogenic section has negligible moisture content.…”

“…We greatly appreciate the careful review carried out by Variny et al [1] and their valuable feedback, which could prove helpful in future studies of air separation units. The comments of Variny et al [1] are summarized in the following:…”