Question

In a distillation operation, what is the effect of the temperature of the reflux stream (given below) on the condenser and reboiler loads? Reflux conditions - Reflux stream is completely liquid and is at its bubble point - Reflux stream is below its bubble point

• A. Condenser and reboiler loads are the same in both the cases
• B. Reboiler load is same in both the cases but condenser load is higher in case 2
• C. Condenser load is same in both the cases but reboiler load is higher in case 2
• D. Both condenser and reboiler loads are higher in case 2 compared to case 1

Hint:

In distillation, the reflux stream temperature plays a crucial role in determining the energy required for both the condenser and reboiler. A subcooled reflux increases reboiler load.

Answer 1

The Correct Option is C

Step 1: Understanding reflux conditions.
When the reflux stream is below its bubble point (i.e., in the subcooled liquid state), it requires more heat to vaporize and increase the amount of vapor in the system. This increases the reboiler load, as more heat is required to generate vapor from the liquid in the reboiler. However, the condenser load remains relatively constant because the condenser’s primary function is to condense the vapor into liquid, which is not significantly affected by the subcooled reflux stream in this case.
Step 2: Conclusion.
The correct answer is (3), where the reboiler load is higher in case 2 compared to case 1, but the condenser load remains the same in both cases.

Answer 2

This question compares two reflux conditions: (1) reflux liquid returned exactly at its bubble point, and (2) reflux liquid returned subcooled (below its bubble point). Let's evaluate what each stream has to do thermally in the two cases.

1. Condenser and reboiler loads are the same in both cases: Subcooling the reflux forces the column to condense extra vapor internally to warm it back up, and this extra vapor must be replenished by the reboiler, so the two loads cannot both stay unchanged between the two cases.
2. Reboiler load same in both cases but condenser load higher in case 2: The condenser's job is simply to condense the overhead vapor to saturated liquid, a duty fixed by the overhead vapor flow and its latent heat; the larger and more consequential impact of subcooled reflux is inside the column itself, not on the condenser.
3. Condenser load same in both cases but reboiler load higher in case 2: The condenser's duty stays essentially the same in both cases since it's set by the overhead vapor rate and its latent heat, which don't change between the two reflux conditions. But when subcooled reflux enters the top tray, it first has to absorb sensible heat from the vapor rising past it before any of that vapor can be considered "returned" as effective reflux, meaning more vapor must condense on the way down, which in turn means more vapor must be generated at the reboiler to keep the internal traffic and separation the same, raising the reboiler load.
4. Both condenser and reboiler loads are higher in case 2: The condenser's duty is governed by the overhead vapor condensing to its bubble point, which is unaffected by what happens to the liquid afterward, so the condenser load doesn't rise in case 2.

Subcooled reflux increases the internal vapor and liquid traffic inside the column, raising the reboiler duty, while the condenser's duty stays the same.

Therefore, the correct answer is condenser load is the same in both cases but reboiler load is higher in case 2.