Question 1

On a tray in a distillation column, how can the liquid gradient (variation in liquid height from inlet to outlet) be minimized to promote uniform contacting and avoid local weeping or entrainment?

Accepted Answer

Introduction / Context: Liquid gradient across a tray can impair mass transfer by creating maldistribution. High gradients increase froth depth near the outlet, raise pressure drop, and promote entrainment, while low-depth regions near the inlet can weep. Practical design features mitigate gradients and promote even liquid residence and vapor–liquid contact. Given Data / Assumptions: Tray types: sieve, valve, or bubble-cap. Conventional operating range below flooding. Large-diameter columns can suffer greater maldistribution without special layouts. Concept / Approach: Liquid gradient is driven by hydraulic head losses along the flow path. Designers can reduce driving velocity, shorten path length, and adjust internals (weir height, inlet/skirt clearances) to flatten the profile. For large shells, multi-pass (split or radial flow) trays reduce distance traveled and distribute load more uniformly. Step-by-Step Solution: Increase weir or adjust skirt to tune froth height at inlet/outlet (Option A).Reduce cap rows traversed and/or liquid velocity; shorten path across deck (Option B).For large diameters, use split/radial/cascade flow to minimize path length (Option C).Combining these measures minimizes gradient—thus Option D. Verification / Alternative check: Tray hydraulics models (backup height, pressure drop, and weir correlations) predict reduced gradient when path length and velocity are decreased and when weir/clearances are optimized. Why Other Options Are Wrong: Individual measures (A, B, or C) help, but best practice is applying them together as needed.None of the above: contradicts established tray design guidelines. Common Pitfalls: Overcorrecting with excessively high weirs causing undue backup.Ignoring vapor distribution; vapor maldistribution can also create gradients. Final Answer: All of the above

Question 2

In scale-up using similarity principles, which forms of similarity should be considered to preserve dynamic behavior from model to full scale?

Accepted Answer

Introduction / Context: Engineering scale-up relies on similarity analysis to ensure that the physics in a laboratory or pilot unit translate to commercial size. Different layers of similarity address shape, motion, and force balance, which must be preserved to maintain comparable performance. Given Data / Assumptions: Model and prototype operate under the same physical regime (e.g., turbulent flow, gravity-driven, etc.). Material properties either matched or accounted for via dimensionless groups. Concept / Approach: Geometrical similarity: all linear dimensions scale by the same ratio, preserving shape and relative placement. Kinematic similarity: velocity fields are similar; matching Reynolds, Froude, or other kinematic groups as appropriate. Dynamic similarity: force ratios are preserved so that governing dimensionless numbers (Re, Fr, We, Pe, etc.) match, ensuring similar momentum/energy balances. Step-by-Step Solution: Define the process regime (e.g., mixing dominated by inertia and gravity).Identify controlling dimensionless numbers (e.g., Re, Fr).Choose scale factors to keep these numbers equal model-to-prototype.Maintain geometry (impeller-to-tank ratio, nozzle locations) to keep boundary conditions comparable. Verification / Alternative check: When similarity is achieved, performance metrics such as heat/mass transfer coefficients or mixing times scale predictably; discrepancies indicate a broken similarity (e.g., unmatched Re due to viscosity differences). Why Other Options Are Wrong: Single-type similarity alone rarely preserves full behavior; flow and forces will differ.Ignoring similarity entirely leads to poor predictions in pressure drop, power draw, and transfer rates. Common Pitfalls: Attempting strict similarity when multiple regimes coexist; compromises may be necessary.Overlooking property changes (e.g., viscosity with temperature) that break similarity. Final Answer: All of the above: geometrical, kinematic, and dynamic similarity

Question 3

In shell-and-tube exchangers, tubes are attached to the tube sheet (a thick, circular plate) either permanently (rolling and brazing) or removably (ferrules). As a practical rule, the tube sheet thickness is normally about what fraction of the tube …

Accepted Answer

Introduction / Context: The tube sheet anchors tubes and seals the shell-side from the tube-side. Its thickness must resist ligament stresses around tube holes, differential pressure loads, and bending from channel/shell connections. Practical sizing rules offer a starting point before detailed mechanical analysis (e.g., TEMA/ASME). Given Data / Assumptions: Conventional shell-and-tube exchanger with straight tubes. Moderate differential pressures and standard tube pitch patterns. Fabrication practice requiring a minimum machinable thickness. Concept / Approach: A common rule-of-thumb sets tube sheet thickness on the order of the tube outside diameter, subject to a practical minimum (≈ 22 mm) for structural integrity and machining. Final thickness is refined by code calculations considering pressure, temperature, gasket seating, and tube-to-tubesheet joint configuration (fixed, floating, U-tube). Step-by-Step Solution: Start with baseline: t_ts ≈ OD_tube.Check against minimum shop limit: t_ts ≥ ~22 mm.Perform code checks (ligament efficiency, bending) and adjust as required.Confirm compatibility with tube expansion/brazing or ferrule details. Verification / Alternative check: Comparisons to TEMA guidelines and past proven designs typically place tube sheet thickness around the tube OD for many standard services, increasing for high pressure or large spans. Why Other Options Are Wrong: Half or three-fourths: often too thin for structural and machining needs.1.5 times OD: overly conservative for routine services, raising cost and weight.Fixed 10 mm: ignores OD and load effects. Common Pitfalls: Using the rule-of-thumb without code verification for high-pressure exchangers.Ignoring gasket seating stresses that govern required thickness. Final Answer: About equal to the tube OD (but not less than ~22 mm)

Question 4

For a rotary dryer, what is the typical cylindrical shell plate thickness used in practice (order-of-magnitude range)?

Accepted Answer

Introduction / Context: Rotary dryers handle granular or particulate solids while rotating a slightly inclined steel shell. The shell thickness must withstand mechanical loads from rotation, support rings, and lifters, while resisting local wear and temperature-induced stresses. Given Data / Assumptions: Medium-size industrial rotary dryer with standard operating temperatures. No extreme pressure differential; operation near atmospheric pressure. Carbon steel construction with typical wear allowances. Concept / Approach: Practical fabrication and structural requirements make ultra-thin shells (2–3 mm, 6–8 mm) unsuitable due to buckling and local denting risks. Excessively thick shells (18–22 mm or more) add cost and weight and are reserved for very large diameters or harsh mechanical conditions. Typical dryers fall in the mid-range, commonly around 12–15 mm. Step-by-Step Solution: Assess loads: dead weight, product holdup, riding rings, and drive.Consider temperature and wear: allow a corrosion/wear margin.Select practical plate thickness: 12–15 mm for common sizes. Verification / Alternative check: Vendor designs and industry references list shells in the low-teens of millimeters for standard dryers, increasing with diameter and duty severity. Why Other Options Are Wrong: 2–8 mm: too thin for structural stiffness and durability.18–22 mm: heavier and costlier than needed for typical units.30–40 mm: reserved for unusual heavy-duty applications. Common Pitfalls: Ignoring local wear under lifters and at feed/discharge—liners may be needed.Not accounting for ovality from ring support spacing. Final Answer: 12–15 mm

Process Equipment and Plant Design Questions