Vessels, fouling, and trays: Identify the WRONG statement among the following equipment-related assertions.

Introduction / Context:
Process equipment design involves standardized components and well-understood hydraulics. This question mixes vessel closures, fouling tendencies of waters, and tray internals to test conceptual understanding and ability to spot an incorrect claim.

Given Data / Assumptions:

• Pressure vessels above atmospheric pressure require suitable heads.
• River water vs. sea water fouling tendency comparison.
• Bubble-cap tray construction: riser (chimney), cap, slots, downcomer, weir.

Concept / Approach:
Formed heads (e.g., hemispherical, ellipsoidal, torispherical) are standard for pressurized cylindrical vessels. Sea water generally has higher fouling/scaling propensity than river water due to salts and biological content, so river water’s fouling factor being lower is reasonable. On bubble-cap trays, risers carry vapour from the tray deck to beneath the cap; liquid flows across the tray and into downcomers. Therefore, saying risers facilitate both liquid and vapour flow is incorrect.

Step-by-Step Solution:
Check (a): True — formed heads are standard for pressure vessels.Check (b): True — sea water tends to foul more than river water.Check (c): False — risers conduct vapour only; liquid does not flow through risers.Hence, the WRONG statement is (c).

Verification / Alternative check:
Tray internals diagrams show vapour rising through risers under caps while liquid flows over the active area and weirs to downcomers.

Why Other Options Are Wrong (i.e., not the incorrect statements):
(a) and (b) are consistent with standard practice. (d) claims both (b) and (c) are wrong, which conflicts with (b) being correct.

Common Pitfalls:

• Confusing risers with downcomers; only downcomers carry liquid between trays.
• Assuming all waters foul similarly; salinity and biology matter.

Final Answer:
Risers on bubble-cap trays in distillation columns facilitate the flow of both liquid and vapour.