Sieve tray design guidelines: identify the incorrect statement regarding weir height, downcomer sizing, calming sections, and downcomer residence time for foaming vs. non-foaming services.
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AIncreasing weir height can improve tray efficiency but increases pressure drop; typical weir heights are about 40–90 mm for pressure columns and 6–12 mm for vacuum service.
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BFor segmental downcomers, a chord length of approximately 60–80% of column diameter and an initial downcomer area of about 12% are reasonable starting points.
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CCalming zones near inlet and outlet are often sized at roughly 75 mm for columns below 1.5 m diameter and 100 mm for larger columns.
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DMinimum recommended downcomer residence time is the same for foaming and non-foaming liquids and is about 20 seconds.
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ENone of the above statements is incorrect.
Answer
Correct Answer: Minimum recommended downcomer residence time is the same for foaming and non-foaming liquids and is about 20 seconds.
Explanation
Introduction / Context:Successful sieve-tray column design requires balancing hydraulics (downcomer capacity, froth height, pressure drop) with mass-transfer efficiency. Several practical heuristics are used for first-cut sizing. This question tests recognition of a commonly misstated rule concerning downcomer residence time across different foaming behaviours.
Given Data / Assumptions:
- General refinery/petrochemical services with conventional sieve trays.
- Statements reflect common preliminary design rules rather than final detailed design.
- Foaming tendency of the liquid affects recommended residence time.
Concept / Approach:Weir height: Higher weirs can enhance liquid contacting time and froth depth but raise pressure drop; typical ranges differ for pressure vs. vacuum columns. Downcomer geometry: Segmental downcomers often occupy around 10–15% area; an initial 12% is a standard starting point. Calming (inlet/outlet) zones maintain uniform distribution. Crucially, downcomer residence time is not “one-size-fits-all”; foaming services typically need longer times than non-foaming services, and 20 s is far above the usual “minimum” guidance used for many systems.
Step-by-Step Solution:Evaluate each statement against standard heuristics.(a), (b), and (c) match accepted practice.(d) is incorrect because residence time depends on foaming tendency and typical minimums are several seconds (often 3–10 s), not a flat 20 s for all services.
Verification / Alternative check:Design texts and vendor guidelines consistently recommend longer residence times for foaming liquids and shorter for non-foaming, with typical minimums well below 20 s for many applications.
Why Other Options Are Wrong:(a), (b), and (c) are broadly consistent with common practice. (e) is wrong because (d) is indeed incorrect.
Common Pitfalls:Confusing “minimum” residence time with “typical” or “conservative” hold-up; ignoring how foam, surface tension, and viscosity alter downcomer sizing and residence requirements.
Final Answer:Minimum recommended downcomer residence time is the same for foaming and non-foaming liquids and is about 20 seconds.