An explosion in a sulphonation plant

The incident occurred in an electrostatic filtration unit of a sulphonation plant. When flushing with air during the start-up phase, an explosion occurred in the upper part of the plant, at the electrostatic precipitators. The most serious effects of the pressure wave were the violent detachment of the manhole cover of the plant, the rupture of two insulating supports and the deformation of the gas distributor plate.

[DECHEMA]

The INITIATING CAUSE was the formation of an explosive atmosphere containing hydrogen.

Moisture-bearing and corrosive gases entered the electrostatic filtration plant during the start-up phase and during operation.
Air from an air dryer is used for cleaning, to prevent deposition of corrosive and conductive moisture on the thermo-protective insulation of the upper part of the filtration plant and lines. Hydrogen was probably generated in the converter, in the inlet moisture separator and/or in the acid scrubber (absorber) by the reaction of iron with sulphuric acid.
The ignition source of the hydrogen / air mixture were electrical discharges observed in the ESP.

This suggests a ROOT CAUSE related to poor control of the process parameters and to design shortcoming.

sulphonation plant, manhole, electrostatic precipatators

At the moment of the incident, the sulphonation plant was starting -up and flushed with air. It is probable that this start-up was part of the production cycle, to clean the system.

In organic chemistry, aromatic sulfonation is a reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid (−SO2OH) group. Together with nitration and chlorination, aromatic sulfonation is a widely used electrophilic aromatic substitutions. Aryl sulfonic acids are used as detergents, dye, and drugs.
Typical conditions involve heating the aromatic compound with sulfuric acid:
C6H6 + H2SO4 → C6H5SO3H + H2O

The report of this event does not specify the products of this plant/unit.
Wikipedia mentions among applications surfactants, detergents, dyes, and pharmaceuticals.

[Wikipedia https://en.wikipedia.org/wiki/Aromatic_sulfonation ]

According to the ProcessNet database [DECHEMA event no. 05/ 2001], the handling of sulphuric acid in the presence of certain metals, e.g. iron and moisture, can produce hydrogen. If the hydrogen is allowed to accumulate, it can form potentially explosive hydrogen / air mixtures with a very low ignition energy. In the specific case of the electrostatic filtration plant, it is necessary to ensure that the hydrogen concentrations remains below the explosive limit before starting up the plant. The flushing has to be performed for a sufficiently long time and its effectiveness confirmed by concentration measurements.
A typical reaction is: Fe (s) + H2SO4 (aq) → FeSO4 (aq) + H2 (g)