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[johneyissac98]

Can anyone explain what is the purpose of giving cold reflux and hot reflux in same distilled column ?

[SilverShaded]

Specifically on CDU units double ovehead drum configuration with hot reflux was an attempt to reduce or prevent corrosion at the top of the column.  When cold reflux is returned to the column localised water condensation can occur which may become strongly acidic due to H2S and cause localised corrosion.  Hot reflux helps prevent this by being well above the water dew point.  Top pumparounds with cold return temperatures have the same issue.

There may still be some cold reflux depending on the heat balance around the two sets of condensers.  Ideally it should be mixed with the hot reflux before entering the column.