Chemical Engineering - Mass Transfer - Discussion

(C) The correct answer is due to:

When the feed condition to a fractionating (distillation) column changes from saturated vapor (q = 0) to saturated liquid (q = 1), it affects the internal flow rates in the column (like liquid and vapor flows in rectifying and stripping sections), but not necessarily the number of ideal stages, if:
The desired separation remains the same (i.e., same xD​ , xB​ , and xF​ )
And, the reflux ratio is kept constant.

The given answer is correct. (a).

A saturated liquid feed is less volatile compared to a saturated vapour feed, which requires more stages to achieve the same separation. Therefore, the feed stage is typically located higher in the column for a liquid feed, increasing the total number of stages needed.

A is the correct answer.

When the feed change from saturated vapour to saturated liquid phase, the reflux will decrease since vapour from the bottom will decrease.

So, for minimum reflux = Maximum plates.

Please explain the answer.

Option C is correct.

Because the reflux ratio and separation remain unchanged. And feed flowrate defines tower diameter.

When feed composition increased and reflux remained unchanged stages will increase as per separation requirement.