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. The process implied the hydrogenation of di-nitrotoluene in presence of isopropanol as solvent and Raney nickel as catalyst. Hydrogen and the other reacting components were injected into a reactor at high pressure.
A run-away reaction occurred due to the mixing of too much reactive product into the reactor. This is an event where hydrogen was involved, but did not have the principal role.

Internal fire-fighters had the blaze under control in 35 min.
Four employees were hospitalised. 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 were damaged.

The INITIATING CAUSE was a runaway hydrogenation reaction due to the injection of too much reacting product into the reactor.

The concentration of DNT in the reactor was insufficiently diluted. This allowed the development of a reaction reaching pressure and temperature beyond their design values.

The injection of too much DNT was due to two partially open valves, allowing 500 to 700 kg/hr of product to flow into the reactor. The heat release from the 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.
The ROOT CAUSE is related to an inadequate safety-design and to the failure of the management to recognise the hazards during operation.

hydrogenation of DNT reactor unit

The accident occurred after that the plant had been taken off-line for scheduled maintenance.

DESCRIPTION OF THE SUBSTANCES INVOLVED:
Raney nickel, also called spongy nickel, is a fine-grained solid composed mostly of nickel derived from a nickel–aluminium 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

3 injuries and one fatiality, the technician handling valves to wash the hydrogenation reactors.

The workshop was destroyed: the reactor burst, the bunker housing the workshop was deformed by the blast wave combined with sprayed fragments, the reinforced concrete wall were cracked open a the control room was damaged.

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 plant’s emergency basin. The wind dispersed the gaseous pollutants released (CO2, CO, NOx, unburned organic matter).

The environmental impact proved to be limited.

Subsequent to this accident, the zone surrounding the bunker was closed to access. This prohibition was then extended to the covered parts of annex buildings, where materials were at risk of falling, as well as to the distillation section adjacent to series "A".
With judicial expert agreement, the prohibition was lifted, by virtue of court order, and after removal of the unstable material supply. The bunker was reopened following reinforcement of its upper structure by a metal frame along with installation of a concrete access tunnel.
The distillation section, exposed to the onset of fire, was also closed for safety reasons with a curtailment of all transfers and heating.

(A) LESSONS RELATED TO 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.

(B) LESSONS RELATED 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.

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