Explosion and fire in chemicals production plant

The accident occurred in the organo-metallic compound production unit, on a transport container for aluminium alkyls, which exploded during a cleansing process at the washing station. The incident was caused by a pressure build-up inside the container caused by flammable gases, mainly hydrogen, formed by the chemical reaction of aqueous caustic soda solution with the aluminium alkyl residues.

This cleansing of the transport containers is a standard procedure after return from customers and before a new filling. However, in this particular case, even after several attempts it was impossible to completely remove the deposits on the inner wall of the container. Tests aiming at the detection of reactive alkyl residues showed negative results. Thus, the deposits were exposed to an aqueous solution of caustic soda (NaOH). Since no production of heat or gases had been detected over a period of several hours, to increase the effectiveness of the cleaning procedure the container was sealed (against the instruction) and kept revolving around its longitudinal axis on a roller system.
After a short time of this process, the container burst, torn apart along the longitudinal axis, while part of is bottom was blown away and damaged a heptane pipeline. The released heptane ignited, and the whole building caught fire.
The container had been sealed infringing operating instructions.
The substances involved were aluminium, aluminium alkyls, heptane (H₃C₅CH₃), sodium hydroxide (NaOH), hydrogen.

The INITIATING cause was the pressure build-up inside the container by formation of flammable gases, mainly hydrogen.
The container had been tested for reactive material, as prescribed by the procedure.
However, the container had been sealed, against what prescribed by operating instructions. This could be the ROOT CAUSE, suggesting a lack of technology/operative knowledge of the employees.

chemicals transport container, heptane pipeline

The event occurred after various failed attempts to perform the cleaning process according to instruction.

ALUMINIUM ALKYS
Aluminium alkyls are organoaluminium compounds generally represented by the formula
AlR3 (or dimers Al2R6), where R is an alkyl group like methyl (-CH3) or ethyl (-CH5).

The most common is the trimethylaluminium (TMA) and the triethylaluminium (TEA).

TMA has the formula Al2(CH3)6 (abbreviated as Al2Me6, where Me stands for methyl). It is an industrially important compound, used as a catalyst or in semiconductors processing.
It is a colourless liquid, pyrophoric and highly reactive, air- and moisture sensitive. It must be strictly handled in inert gas atmosphere. It can decompose spontaneously in air to give aluminum oxide in a very exothermic reaction. To eliminate the residues contained in the tank, a reaction with NaOH was performed, which is exothermic and produce aluminium oxide and methane.

TEA (Al2Et6 or Al2(C2H5)6 ) is used as reagent in organic chemistry and as pyrophoric agent. In contact with water ir produces H2 and CH4 in approximated ratio 1 to 2.

According to ARIA report, the injured persons were two.
According to ZEMA , only 1.
The material damage were assessed around 3,6 M de DM

This is an accident where hydrogen was produced accidentally, by reaction of chemical product; therefore, in general, measures aiming at avoiding recurrence of the same type of accidents should concentrate in preventing formation of explosive atmospheres. These could be based on a ventilation purge strategy, and/or on the choice of less reactive cleansing substances allowing for a slower, thus better controllable reaction dynamics.

According to the sources (see References), the following measures were recommended:
(1) Cleaning operations of the containers to be performed with a washing medium not reacting with deposits in the containers;
(2) To provide the washing station with weighing systems, allowing a determination of residue quantities after washing by weighing the container before and after the cleaning process;
(3) To review of the operating instructions.
(4) To performing safety training activities.
(5) To develop accident scenarios and modelling their consequences;
(6) To review the emergency plans.