CHEMICAL ENGINEERING TRANSACTIONS VOL. 35, 2013 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Varbanov, Jiří Klemeš, Panos Seferlis, Athanasios I. Papadopoulos, Spyros Voutetakis Copyright © 2013, AIDIC Servizi S.r.l., ISBN 978-88-95608-26-6; ISSN 1974-9791 Inert Substances and Explosion Limits of Hybrid Mixtures Jiří Serafín *,a , Miroslav Mynarz b , Petr Lepík a , Jana Drgačová b a VŠB – Technical University of Ostrava, Faculty of Safety Engineering, Department of Safety Management, Lumírova 13, 700 30 Ostrava – Výškovice, Czech Republic b VŠB – Technical University of Ostrava, Faculty of Safety Engineering, Department of Fire Protection, Lumírova 13, 700 30 Ostrava – Výškovice, Czech Republic jiri.serafin@vsb.cz Problems of formation of hybrid mixtures are a rather neglected hazard to the manufacturing process. With the origin of such mixtures a necessity of minimizing possible risks that may accompany the formation of hybrid mixtures is connected as well. The article focuses especially on problems of possible inerting of hybrid mixtures. 1. Introduction At present, when new substances are used, the formation of hybrid mixture is not any great problem; that appears at the moment when a possibility of formation of hybrid mixture is neglected and thus a hazard resulting from the formation of hybrid mixture is not handled sufficiently. One of possibilities of preventing large-scale damage that can be caused by the occurrence of hybrid mixture is efficient inerting of this mixture. 2. Definition of used basic terms 2.1 Lower explosion limit The lower explosion limit (henceforth referred to as LEL) is the lower limit of explosion range. LEL represents such value when a shortage of flammable substance in the mixture with an oxidizing agent manifests itself. Here it should be stated that LEL does not depend on a ratio of oxygen to nitrogen in the mixture as stated frequently (Damec, 1998). 2.2 Hybrid mixture The hybrid mixture is a mixture of air and flammable substances of different physical conditions. It should be mentioned here that a small amount of vapours of a flammable liquid or a flammable gas is sufficient for reducing the lower explosion limit of flammable dust-air mixture. The violence of hybrid mixture explosions can be not predicted by simply overlapping the effects of the single (only dust and only gas) explosion. (Russo et al., 2012) 2.3 Inerting Inerting is included in primary explosion protection. In practice this method is increasingly utilized for its efficiency and maintaining safety. The formation of explosive concentrations is prevented by means of inert substances. As inert substances, substances in gaseous state (e.g. nitrogen, carbon dioxide, noble gases, halohydrocarbons), water vapour and admixtures of solid inert substances in powder state can be used. Disadvantages of this method consist in higher operational costs. In practice, gaseous nitrogen is the most used substance for inerting. In the course of gas inerting, the space in which the inert is to be used is necessary to be enclosed. Nitrogen is capable to ensure a rapid increase in minimum initiation energy and a drop in maximum explosion parameters in the system. If it reaches the limiting oxygen concentration (LOC) value, the mixture cannot be initiated by the initiation source with standard energy of 10 J any more. If the initiation energy is higher (flame, electric arc, etc.), initiation may occur even at LOC value. On the DOI: 10.3303/CET1335186 Please cite this article as: Serafin J., Mynarz M., Lepik P., Drgacova J., 2013, Inert substances and explosion limits of hybrid mixtures, Chemical Engineering Transactions, 35, 1117-1122 DOI:10.3303/CET1335186 1117