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Distillation column melts and blocks

The Incident

A loss of still vacuum on a chlorinated phenol plant was investigated and it was found that not only was the distillation column was totally blocked with "coke", but the top of the column had partly melted.

The standard practice was to let down the vacuum at the end of a distillation with air.  The temperature at this time was unknown, as the recording stopped with the end of the batch - the last recorded temperature was 250°C.

The Investigation

Thermal stability tests of the still residues and product were inconclusive, there appeared to be insufficient energy to cause such an incident.  However it was seen that the still residue of the final batch was extremely hard, suggesting that the distillation may have been forced a little more than normal, this would have made the column that little bit warmer than normal.  Thus when the air rushed in there was opportunity for a rapid oxidation which resulted in the column melting and the formation of the solid in the column.

Recommendations

In view of the problems encountered, although only on one batch, it was recommended that in all future batches, the vacuum be let down with nitrogen

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