Actual operation of a bubble-cap distillation column: which variables are higher at the bottom than at the top of the tower?

Introduction / Context:
Distillation columns operate with thermal and hydraulic gradients from bottom (reboiler) to top (condenser). Understanding which variables increase or decrease along the height helps diagnose performance, set controls, and interpret composition profiles.

Given Data / Assumptions:

• Standard fractionation tower with a reboiler at the bottom and a condenser at the top.
• Countercurrent vapor–liquid contact on trays or packing.
• No unusual feeds that invert normal gradients.

Concept / Approach:
Because of static head and pressure drop through internals, the bottom operates at higher pressure than the top. The reboiler adds heat, so temperatures are higher at the bottom and decline toward the top where heat is removed in the condenser. While vapor molecular weight can vary with composition, it is not universally higher at the bottom for every system. Similarly, actual molar vapor flow can vary by section and heat balance and is not guaranteed to be greater at the bottom than at the top in all operations.

Step-by-Step Solution:

Verification / Alternative check:
Process data trends and tower profiles consistently show bottom temperature/pressure maxima for typical hydrocarbon separations.

Why Other Options Are Wrong:

• Molal vapor flow and vapor molecular weight depend on compositions and energy balances; they are not always higher at the bottom.
• “All of the above” is therefore not generally true.

Common Pitfalls:
Assuming constant molar overflow without checking energy input/removal; generalizing composition trends to every system.

Final Answer:
Pressure and temperature