Explosion at the Toluene Di-Amide Synthesis Unit of a Chemical Plant
An explosion and fire occurred at night inside a unit for the synthesis of toluene di-amine (TDA). The process implies the hydrogenation of di-nitrotoluene (DNT) in presence of Isopropanol as solvent and Raney nickel as catalyst. Hydrogen and the other reacting components is injected into a reactor at high pressure. Internal fire-fighters had the blaze under control in 35 minFour employees were hospitalised. The technician handling valves to wash the hydrogenation reactors with isopropanol sustained burns to 40%-50% of his body and died 15 days later. The plant was destroyed. Glass panes shattered over a 50-100 m radius; the distillation unit adjoining the bunker was also damaged, causing the isopropanol and TDA to leak out, thus exacerbating the fire outbreak. Buildings at the neighbouring industrial site 150 m away had their lightweight structures deformed. Extinction water was channelled to the plants emergency basin. The wind dispersed the gaseous pollutants released (CO2, CO, NOx, unburned organic matter). The environmental impact proved to be limited.
Event Date
December 27, 1995
Record Quality Indicator
Region / Country
Event Initiating System
Classification of the Physical Effects
Nature of the Consequences
Cause Comments
There was an insufficiently diluted DNT concentration in the reactor. Reaction conditions, as verified in the laboratory, caused the pressure and temperature rise required to trigger the accident. This was caused by two partially open valves allowing 500 to 700 kg/hr of product to flow into the reactor. The heat release from hydrogenating a small quantity of DNT most likely triggered, through sudden heating of the reaction medium, the violent decomposition of the remaining DNT.The investigation discovered that risk analysis performed for this plant was inadequate.
Facility Information
Application Type
Application
Specific Application Supply Chain Stage
Components Involved
hydrogenation of DNT reactor unit
Storage/Process Medium
Actual Pressure
100
Design Pressure
100
Location Type
Location description
Industrial Area
Pre-event Summary
The accident occurred after that the plant had been taken off-line for scheduled maintenance. Info on substances involved:Raney nickel, also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickelaluminum alloy. Several grades are known, of which most are grey solids. Some are pyrophoric, most are used as air-stable slurries. Raney nickel is used as a reagent and as a catalyst in organic chemistry (from Wikipedia)The formula of the toluene di-amine (TDA) is C7H10N2 The formula of the di-nitrotoluene (DNT) is C7H6N2O4
Lessons Learned
Lessons Learned
ORGANISATION AND CONTROL: Risk prevention is based for the most part on the juxtaposition and comparison of procedures established from guidelines and from automated mechanisms intended, at times, to render certain operations more reliable. In the present case, explicit guidelines dating from 2nd May, 1994 were not respected (i.e. no additional DNT during washing operations). Routine, old habits and experience are not justifications for waiving current rules. Moreover, a visual inspection of the entire set of valves should have led to observing the opening of both valves. An extra barrier, such as servo-controlled valves on this DNT line, would have prevented eventual DNT introduction into reactor 901a during the washing sequence prior to the accident. INDENTIFICATION AND EVALAUTION OF RISKS, PROCESS SUPERVISION: In light of the high exothermicity of the DNT hydrogenation reaction, several elements were capable of leading to this accident, namely: - the washing sequence using a discontinuous process, - the installation configuration, particularly the DNT line, - the washing sequence modification subsequent to process improvements (less reactor clogging). This modification, wholly justified yet not transcribed in the guidelines, prevented DNT dilution. All these points led to an insufficiently diluted DNT concentration in the reactor. Reaction conditions, as verified in the laboratory, caused the pressure and temperature rise required to trigger the accident. An assessment focusing on the risks related to opening DNT pipe valves during reactor washing with isopropanol under H2 pressure, along with in-depth knowledge of the process, would have allowed evaluating these risks and then modifying the installation prior to the accident. The risk analysis performed for this plant seems to have been inadequate.TECHNICAL MEASURES: To reduce the probability of occurrence of this type of accidents, the following measures were adopted, aiming at preventing the injection of pure di-nitrotoluene into the reactor: 1.elimination of the DNT intake on the injection tank; 2.addition of 2 automatic on/off valves on the DNT supply pipe at the mix tank; 3.closure of the connection (using an automatic on/off valve) between the mix tank and the injection tank during the reactor washing phase; 4.modification of washing procedures (from a discontinuous to continuous mode); 5.rearrangement of reactor layout to minimise the need for personnel to enter the bunker
Event Nature
Emergency Action
During the fire-fighters' response, water from the plant's collector pipe was diverted to the site's emergency basin until official observation of the absence of amine (i.e. a pollutant characteristic of shop operations).
Detonation
No
Deflagration
No
High Pressure Explosion
No
High Voltage Explosion
No
Source Category
References
References
full report from ARIA data base
event no. 7956
brief description from ARIA no. 7956