Question

Describe, qualitatively, what happens in a distillation column during cold startup (operating under total reflux). Assume that, during start-up, the column is operated with no distillate draw, and little to no bottoms draw (and any withdrawn bottoms are cooled and returned to the feed reservoir).

Answer 1

The total reflux condition represents operation with no product removal. All the overhead vapor is condensed and returned as reflux. Consequently, the reflux ratio (L/D) is infinite. This, in turn, makes the operating lines the 45 degree line (prove it to yourself by setting D=0, and noticing that consequently L=V). With the operating lines on the diagonal, they are as far as they can get from the equilibrium curve, so if the number of plates are stepped off using the diagonal and the equilibrium curve, the number of theoretical stages will be a minimum.

Often, columns are operated at total reflux during their initial startup, and product is not withdrawn until a separation close to that desired is achieved.