An explosion in a sulphonation plant.

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

[DECHEMA incident database, event no. 05/ 2001]

Moisture-bearing and corrosive gases entered the electrostatic filtration plant (ESP) during the start-up phase and during operation of the plant.
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. The cause of the explosion is probably the presence of hydrogen 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 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 unknown is if this start-up was part of the production cycle, or caused by some other abnormal operation.

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)