energies Reply 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” Muhammad Haris Hamayun 1,2 , Naveed Ramzan 1 , Murid Hussain 2 and Muhammad Faheem 1, *   Citation: Hamayun, M.H.; Ramzan, N.; Hussain, M.; Faheem, M. 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”. Energies 2021, 14, 6445. https://doi.org/10.3390/ en14206445 Academic Editor: Noam Lior Received: 7 July 2021 Accepted: 27 September 2021 Published: 9 October 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Chemical Engineering, University of Engineering and Technology, Lahore 54890, Pakistan; mhhamayun@cuilahore.edu.pk (M.H.H.); drnramzan@uet.edu.pk (N.R.) 2 Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Defence Road, Lahore 54000, Pakistan; drmhussain@cuilahore.edu.pk * Correspondence: faheem@uet.edu.pk; Tel.: +92-333-4699032 1. Introduction 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 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: • Model assumptions as formulated in Hamayun et al. [2] are incomplete. The pressure drop in heat exchangers assuming a constant value of 10 kPa, regardless of the position in the process scheme is unjustified, similarly to assuming zero pressure losses. • Hamayun et al. [2] omitted pressure losses in adsorbers. The issue of pressure loss in adsorbers is of serious concern and can be subject to optimization. It contributes to energy consumption of the air separation unit and should thus be considered. • Adsorbers are modeled as component splitters, which assumption is over-simplified. The effect of water steam adsorption heat should be considered as it may reach up to 3000 to 4000 kJ/kg of adsorbed steam for conventional zeolites used in compressed air drying by adsorption. The resulting temperature increase in air passing through adsorbent layer can thus exceed 10 or even 20 ◦ C depending on the water steam content in the inlet air which, in turn, impacts the equipment downstream. • The energy consumption evaluation is incomplete as it does not incorporate energy needed for adsorber regeneration. As mentioned above, significant amount of heat is released by steam adsorption on adsorbent and thus its regeneration is energy intense and contributes to the overall energy consumption of the air separation plant. Heat recuperation is often proposed to cut down the adsorbent regeneration cost which, however, adds another complexity to the plant. • Moist air cooling in multi-stream heat exchanger directly to –100 ◦ C and below after its intake from ambient environment as depicted in process scheme C7 is technically infeasible. It would lead to ice formation and air path blockage, possibly followed by heat exchanger damage. • The importance of using a proper thermodynamic package should be addressed. Peng–Robinson equation is recommended for applications comprising nonpolar gases and vapors, which holds true for nitrogen and oxygen, or dry air but certainly not for water steam. 2. Our Replies At the outset, we would like to re-emphasize the scope of our study by quoting from our published work [2]: Energies 2021, 14, 6445. https://doi.org/10.3390/en14206445 https://www.mdpi.com/journal/energies