Assessment on thermal hazards of reactive chemicals in industry: State of the Art and perspectives

Abstract

Thermal hazards of reactive chemicals have been a major concern due to the unceasing occurrences of fire and explosion accidents in industry. Understanding thermal threats of these chemicals not only contributes to the process safety and sustainability in the research and development (R&D) level, but promotes the efficiency of loss prevention, firefighting and emergency responses. Numerous studies have been conducted towards the comprehensive assessment of thermal hazards of reactive chemicals. The chemicals and methods varied in these studies, and yet some topics were commonly concerned by researchers in relevant fields. Further to say, these topics should also be targeted at when dealing with thermal hazards of new chemicals in future. This article provides an up-to-date overview of the common ground regarding the comprehensive understanding of thermal hazards of reactive chemicals in industry level; whereby, the main limitations and challenges to be faced are explored. The discussed key points include, the classification and reactivity of typical reactive chemicals, the fundamental steps towards the comprehensive understanding of their thermal hazards (including the identification of reaction mechanisms, the calculation of reaction kinetics and thermodynamics, and the characterization of thermal safety properties), and the applicable theoretical, experimental and engineering approaches in each step. The primary goal of this review is to lay out the essential basics that should be focused in every trial to comprehensively understand the thermal hazards of reactive chemicals. The further research directions are also presented based on the current research gaps and the context of Industry 4.0.

Introduction

Due to the boom of modern chemical industry since the 19th century, numerous chemicals have been developed. Chemical substances with potential adverse effects on people, property and environment are generally known as “hazardous materials”. The term of “reactive chemicals” refers to a category of hazardous chemicals initiating chemical reactions by themselves or with other substances [1]. These reactions are often exothermic and can produce flammable gases and/or explosive products to trigger accidents with severe consequences. Reactivity of these chemicals can be driven by friction, shock, electrostatic charge and heat, and so on. Among these factors, thermal related reactivity is of great concern to the fire protection community. A review of 319 major industrial accidents occurred from 1917 to 2011 indicated that, three cases including fire, explosion and toxic release dominated the accidents in industry [2]. Primary cases could have secondary effects. For example, fires were often followed by explosions and vice versa. A large number of fire and explosion accidents have occurred in chemical facilities, plants, warehouses, and industrial parks or during transport, primarily due to the uncontrolled thermal runaway incidents of reactive chemicals [2], [3], [4].

Although the total number of major industrial accidents has been decreasing in recent years [2], fire and explosion incidents and accidents are still of great challenge to the industry communities for their catastrophic consequences once happened. For example, a major fire and explosion accident [5], [6], [7] occurred at a container storage station of Tianjin Port, China in 2015. This accident caused 165 deaths, 798 injuries, over 6 billion financial losses, disastrous influence on community, and severe pollution to the environment. The initial fire and explosion was caused by an overheated container for nitrocellulose (NC), which is an energetic material with spontaneous combustion hazard. A followed-by more dramatic explosion occurred, mainly due to the detonation of 800 tons of ammonium nitrate (AN). Due to a large amount of other reactive chemicals stored in the same area, fires continued to burn uncontrollably and finally led to 8 additional explosions in the industrial park. This accident has drawn high attention from the industry communities due to its dramatic consequences in the contemporary era. It is apparent that the thermal hazards of NC, AN, as well as other reactive chemicals were under-estimated in industrial level, and there was no effective response plan for the potential thermal runaway.

Back to the very beginning, the lack of accurate understanding of thermal hazards of reactive chemicals is a major cause of almost all the fire and explosion accidents in modern industry [8], [9], [10]. Fig. 1 explains how the thermal hazards of reactive chemicals can contribute to thermal runaway incidents and the followed-by fire and explosion accidents under a general chemical application scenario. Apparently, accurate understanding of chemicals is a prerequisite for their sustainable use at the R&D stage. Scientific knowledge of their thermal hazards can also enable the prediction of unfolded consequences if chemicals are exposed to thermal environment, and thus help with emergency response in industrial parks [11]. Note that the inherent property of chemicals and mixtures is not the only factor that contribute to fire and explosion. A thermal runaway process is generally governed by three factors [12], the heat generation due to reaction (influenced by mass of reactive mixture and reactor size), the heat exchange with environment (such as the surface of reactor), and the internal heat and mass transfer (such as the physical state of substances and thermophysical properties).

Many researchers and engineers in fields of chemistry, fire and explosion, process safety and loss prevention need to deal with chemicals containing potential thermal threats constantly. The comprehensive understanding of their thermal threats is a challenging target due to the complex coupling of both inherent property and environment conditions. Numerous studies have been conducted theoretically and/or experimentally to investigate the thermal hazards of the big reactive chemical family. We have analyzed existing studies regarding this topic in the last three decades, and identified the common ground that has been highly focused. Despite the varieties in chemicals and application scenarios, there were some fundamental actions towards the purpose of comprehensive thermal hazard assessment. The thermal reactivity of a chemical must be fully understood through a series of physicochemical parameters. The potential thermal threats of a chemical such as thermal stability, flammability, explosiveness and ignitability (by self-heating or external energy) should be quantitatively characterized. A reactive chemical should also be properly classified on basis of the reactivity phenomena and potential hazards for storage or transport purposes.

To the knowledge of the authors, there is no state-of-the-art review article with regard to the aforementioned actions, which should also be the key points towards the comprehensive understanding of thermal hazards of new chemicals in future. This paper does not intend to target at a particular form of thermal hazard or a certain kind of chemical. The fundamental purpose of this article is to present the issues of interest regarding the common ground with up-to-date discussions, and to provide a guideline for potential users to handle the thermal hazards of a new chemical. Some new perspectives are also presented to fill in the current research gaps. Note that the term of thermal hazard technically differs from thermal risk. The term of risk or the likelihood of harm occurring is introduced when exposure is considered in conjunction with the data of potential hazards [12]. This article mainly focuses on the extensive studies related to the thermal hazards of reactive chemicals; nevertheless some contents of thermal risks are also covered due to the intimate relations between hazard identification and risk assessment in industry. It is expected that this review will provide values to readers in relevant fields by seeing the whole picture, and bring helps to investigate the thermal threats of reactive chemicals in future.