Investigation of a violent explosion for nitric acid—n-hexane system using calorimetric method

Abstract

With the acceleration of the energy transition, new synthetic routes for converting alkanes into high-value products are emerging. However, the thermal safety of these new routes may not have been fully considered, potentially leading to dangers during the optimization of reaction conditions. This study reports, for the first time, a reactor explosion incident during the experiment of a new synthetic route for adipic acid: nitric acid oxidation of n-hexane. Differential scanning calorimetry (DSC), accelerating rate calorimetry (ARC), and corrective calculations were used to investigate the cause of the explosion. The results indicate that the polyparaphenol (PPL) liner material, which was used for the first time in the experiment, is unlikely to react with the system and cause the explosion. When the nitric acid concentration is increased from 3.7 mol·L−1 to 5.4 mol·L−1, the thermal stability of the system decreases, and the heat release surpasses the chemical explosion threshold (1000 J·g−1). The maximum self-heat rate (dT/dtmax) increases by at least 7 times, and the maximum pressure rise rate (dp/dtmax) increases by at least 11 times. This led to the actual pressure in the reactor increasing from 4.96 MPa to at least 11.09 MPa, which far exceeded the rated pressure (3 MPa) and reached the rupture pressure (3.5 to 4 times their rated pressure), resulting in the explosion. This study aims to provide a warning regarding the safety of new synthetic routes involving the nitric acid−organic systems, particularly the conversion of alkanes, to prevent the recurrence of similar incidents.

1. Introduction

The global efforts toward carbon neutrality are accelerating the shift from fossil fuels to renewables, driving an expected oil supply surplus by 2030 [1]. Meanwhile, the demand for adipic acid, a crucial chemical intermediate used extensively in the production of Nylon 66 and engineering plastics, is increasing [2]. However, the production capacity of adipic acid has been limited due to the complexity of the preparation process for its raw material, cyclohexane. As a result, a selective oxidation route is being explored to convert petroleum alkanes, such as n-hexane, into adipic acid at elevated temperatures. In this process, nitric acid could be employed as the oxidizing agent, with adipic acid being the primary oil product. However, the introduction of this new synthetic route could pose novel risks. For example, the proposed method for synthesizing adipic acid through the oxidation of cyclohexene with hydrogen peroxide [3] was criticized for its highly exothermic nature and the low thermal stability of hydrogen peroxide, which rendered the process hazardous [4—6]. Similarly, nitric acid, a widely used industrial nitrating and strong oxidizing agent, generates large amounts of heat and gas (pressure) when reacting with organic substances, increasing the potential for runaway reactions, even explosions [7]. The concentration of nitric acid is a critical factor that influences this risk.

Nitration processes are well known for their thermal hazards, with nitric acid concentration being a key factor in determining nitration safety. Liu et al. [8] found that higher HNO3 concentrations increased byproduct formation and adiabatic temperature rise in the nitration of N-(1-ethylpropyl)-3,4-dimethylaniline. Murray et al. [9] identified concentration-dependent reaction kinetics and exothermicity in fuming HNO3-mediated aryl boronic acid nitration, underscoring the importance of accurate acid titration. Chen et al. [10] demonstrated that the HNO3/H2SO4 mixed acid system catalyzed thermal decomposition of 2-ethylhexyl nitrate (2-EHN) during iso-octanol nitration, with subsequent studies revealing enhanced exothermicity in HNO3/2-EHN mixtures compared to pure 2-EHN decomposition [11]. The oxidation of organic compounds by HNO3 is an autocatalytic reaction characterized by an induction period, during which runaway conditions can be difficult to detect [7]. Thermal analysis using C80 revealed that the rate of autocatalytic reactions in the HNO3-formic acid system increased with the concentration of both acids, indicating that low acid concentrations are crucial for the thermal safety of this system [12]. Kinetic studies on the HNO3 oxidation of 2-octanol demonstrated that the rate constant is highly sensitive to HNO3 concentration [13]. The accidental contact of nitric acid with organic materials has been a longstanding concern in chemical process safety, and this study underscores the need for thorough thermal hazard assessment in novel synthetic routes.