Client Background
During a HAZOP study for a natural product and ingredient manufacturer, the potential inadvertent mixing of two raw materials was highlighted. This scenario was required to undergo a Chemical Reaction Hazards assessment.
Client Problem
A tanker unloading station was to receive concentrated nitric acid (HNO3) and isopropyl alcohol (C3H8O) at different times, a small dead leg was identified in the pipeline meaning residual material could have accumulated here between unloading.
The reaction between nitric acid and isopropyl alcohol is known to be vigorous and has caused previous industrial incidents.
The formation of rocket propellants as a by-product of the reaction further complicates the risk.
Exact reaction pathways and products of this reaction are not clear in the literature, further cementing the importance of reactive testing.
Client Objectives
To determine the pressure and temperature rise rates, and adiabatic temperature rise when the reagents are mixed. This would validate whether existing venting in-situ was sufficient, or if changes to the plant design or process needed to be made.
Strategy
For the test to be representative of the client’s process, the ARSST test cell was assembled under an air atmosphere with the nitric acid contained.
Isopropyl alcohol was measured into the dosing syringe and carefully loaded to the containment vessel ready for analysis of the addition.
The ARSST cell was maintained at ambient pressure with air to best replicate the mischarge scenario, then heated under polynomial control at 2 °C/min to 25 °C and stabilised at this temperature. Use of the polynomial setting even for reagent additions allows for pseudo-adiabatic quantification of reaction enthalpies and temperature rises.
Isopropyl alcohol was measured into the dosing syringe and carefully loaded to the containment vessel ready for analysis of the addition.
The ARSST cell was maintained at ambient pressure with air to best replicate the mischarge scenario, then heated under polynomial control at 2 °C/min to 25 °C and stabilised at this temperature. Use of the polynomial setting even for reagent additions allows for pseudo-adiabatic quantification of reaction enthalpies and temperature rises.
Industrial accident caused by nitric acid and isopropyl alcohol mixing
Insights and results
Sigma-HSE found that
By adopting the CCPS’s RBPS framework, the company moved from a reactive to a proactive safety culture. The company’s commitment to continuous improvement, leadership accountability, and structured risk management positioned it as a safer and more reliable operator.
The testing confirmed that the reaction proceeded in a 2-stage manor, with an intermediate being formed that was still solvent or liquid at low temperatures. The secondary reaction demonstrated much greater risk due to higher reaction enthalpy, rapid temperature rise, and intense permanent gas generation capable of compromising containment.
The results and reporting were used to identify and confirm the worst-case scenario for pressure relief requirements and calculate the vent size for the ERS, supporting the design and validation of a safe process with a clear understanding of the potential risks involved.
