In a distillation column, for a saturated vapor feed, what is the value of q (the fraction of the feed that is liquid)?

Introduction / Context:
The q-line in McCabe–Thiele analysis represents the thermal condition of the feed. The parameter q is defined as the fraction of the feed that is liquid at the feed tray conditions.

Given Data / Assumptions:

• q = liquid fraction of the feed (on a molar basis).
• Feed is saturated vapor (no subcooling, no heat removal needed to vaporize).

Concept / Approach:
By definition, if the feed arrives as saturated vapor, the liquid fraction is zero. Hence q = 0. For saturated liquid, q = 1. For subcooled liquid, q > 1; for superheated vapor, q < 0.

Step-by-Step Solution:
1) Identify feed state: saturated vapor.2) Apply definition: q = liquid fraction of feed.3) Saturated vapor has zero liquid present → q = 0.

Verification / Alternative check:
On the x–y diagram, a saturated-vapor q-line is horizontal (slope = q/(q−1) = 0/(−1) = 0), matching the textbook construction.

Why Other Options Are Wrong:
1 is for saturated liquid, not saturated vapor.< 0 corresponds to superheated vapor.> 1 corresponds to subcooled liquid.

Common Pitfalls:
Confusing thermal condition (q) with phase quality used in other contexts; forgetting that saturated states lie exactly at the dew/bubble curves.

Final Answer:
0