Tray design — For a bubble-cap tray to run stably with even vapour distribution, which of the following statements is incorrect (i.e., the “wrong statement”)?
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AThe pressure drop due to the caps and slots plus static submergence should be as high as practicable for reasonable operation.
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BStability tends to increase by increasing cap skirt clearance, lowering liquid flow per unit plate width, or increasing cap spacing.
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CThe ratio of liquid hydraulic gradient to pressure-drop head across the bubble-cap assembly should be less than 0.4.
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Dnone of these.
Answer
Correct Answer: The pressure drop due to the caps and slots plus static submergence should be as high as practicable for reasonable operation.
Explanation
Introduction:Bubble-cap trays require a balanced hydraulic design. Adequate but not excessive pressure drop is needed to suppress weeping and maldistribution. Selecting the incorrect guideline can lead to high energy consumption, entrainment, and unstable operation.
Given Data / Assumptions:
- Conventional bubble-cap tray with downcomers, risers, and caps.
- Steady distillation service with typical hydrocarbon or aqueous systems.
- Goal is stable operation and uniform vapour distribution.
Concept / Approach:Good practice aims for a reasonable tray pressure drop: high enough to prevent backflow/weeping and to dampen maldistribution, yet low enough to minimise energy penalty and entrainment. Design also considers skirt clearance, cap spacing, and liquid load to maintain froth and residence time without overwhelming the tray hydraulics.
Step-by-Step Solution:Identify that “as high as practicable” pressure drop is undesirable; it increases energy use and can worsen entrainment and flooding risk.A moderate pressure drop (often around 1–2 kPa per tray, service dependent) is typical to achieve stability.Guidelines such as limiting liquid gradient relative to cap pressure drop (e.g., < 0.4) promote even vapour distribution.Adjustments to skirt clearance, cap spacing, and liquid flow per plate width affect froth quality and stability.
Verification / Alternative check:Compare with tray vendor recommendations and design texts: they emphasise “sufficient” pressure drop, not “as high as practicable.”
Why Other Options Are Wrong:
- (b) describes commonly used adjustments for stability (within bounds).
- (c) reflects a recognized dimensionless guideline helping avoid maldistribution.
- (d) is false because (a) is already identified as wrong.
Common Pitfalls:Overdesigning pressure drop; ignoring the coupling between pressure drop, entrainment, and capacity; forgetting that excessive skirt clearance can also induce weeping at low rates.
Final Answer:The pressure drop due to the caps and slots plus static submergence should be as high as practicable for reasonable operation.