Equipment for gas-liquid contact: tray and packed columns
How tray and packed columns work, tray types and operating limits, tray efficiencies, packing types and hydraulics, diameter from flooding velocity, and how to choose between trays and packing.
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Why it matters
Absorbers, strippers and distillation columns are either tray (plate) columns or packed columns. The choice decides capital cost, pressure drop, turndown, ease of cleaning and how the column behaves near its limits. Interviewers and plant engineers expect you to know how each works, how it fails (flooding, weeping, entrainment, maldistribution) and how a diameter is first estimated.
Key ideas
Tray columns – staged contact. Liquid flows across each tray, over an outlet weir and down a downcomer to the tray below; vapour rises through openings in the tray deck and bubbles through the liquid, forming a froth where mass transfer happens. Each tray approximates one equilibrium stage, corrected by an efficiency.
- Sieve trays: simple perforated plates; cheapest, good capacity, but weep at low vapour rates (turndown about 2:1).
- Valve trays: liftable valves close at low vapour rate; turndown about 4:1 or more at moderate cost. The most common choice today.
- Bubble-cap trays: caps over risers keep a liquid seal even at very low vapour rates, so they do not weep; used where very low flows or high turndown are needed, but expensive and with higher pressure drop.
- Downcomers carry liquid down and must give the froth time to disengage vapour; the weir sets the liquid depth on the tray.
Tray operating limits. The stable operating window is bounded by:
- Flooding: excessive vapour rate carries liquid up (entrainment flooding), or liquid backs up in the downcomer until it reaches the tray above (downcomer flooding). Pressure drop rises sharply and separation collapses.
- Excessive entrainment: droplets carried to the tray above reduce efficiency even before flooding.
- Weeping and dumping: too little vapour lets liquid drain through the holes.
- Coning at low liquid rates: vapour jets through the liquid without forming froth.
Tray efficiency.
- Overall efficiency
E_O= theoretical stages / actual trays (typically 0.5–0.8 for distillation, often lower for absorption). Estimated from O'Connell-type correlations or plant data (take from your data book). - Murphree vapour efficiency compares the actual change in vapour composition across a tray with the change if the leaving vapour were in equilibrium with the leaving liquid.
Packed columns – continuous (differential) contact. Liquid trickles down as films over the packing; gas flows up through the voids. Composition changes continuously with height, designed by HTU–NTU (later topic) or by HETP.
- Random packings: Raschig rings (old, low efficiency), Pall rings, Berl and Intalox saddles, modern metal rings – dumped into the shell.
- Structured packings: corrugated metal sheets or gauze arranged in layers – very high area, low pressure drop per stage, excellent for vacuum distillation.
- Internals: a support plate, a good liquid distributor at the top, and redistributors at intervals (commonly every 3 column diameters for rings or about every 6 m) because liquid migrates to the wall.
- Wetting: a minimum liquid rate is needed to wet the packing; below it efficiency falls.
Packed column hydraulics. On a log–log plot of pressure drop per metre against gas rate, the dry-packing line steepens at the loading point (liquid holdup starts to rise with gas rate) and becomes nearly vertical at the flooding point. Design is usually at 60–80 % of the flooding gas velocity, using a generalised pressure-drop correlation (Eckert/Strigle type) and the packing factor F_p from vendor tables.
Choosing between them.
- Packed: smaller diameters (below ~0.6 m), corrosive service (ceramic or plastic packing), foaming liquids, low pressure drop (vacuum), low liquid holdup (heat-sensitive materials).
- Trays: large diameters, high liquid rates, wide flow variations, solids or fouling (trays are easier to clean via manways), side draws and intermediate coolers, and where reliable efficiency prediction is needed.
Formulas
Overall tray efficiency and actual trays:
N_actual = N_theoretical / E_O (round up)
Murphree vapour efficiency for tray n (vapour flows up from n+1 to n):
E_MV = (y_n − y_n+1) / (y*_n − y_n+1)
- y_n – vapour leaving tray n; y_n+1 – vapour entering from below; y*_n – vapour in equilibrium with liquid leaving tray n (mole fractions).
Souders–Brown flooding velocity (based on net area):
u_f = C_SB · √((ρ_L − ρ_V)/ρ_V)
- u_f – flooding vapour velocity, m/s; C_SB – capacity parameter, m/s (from Fair's chart for the tray spacing and flow parameter); ρ_L, ρ_V – kg/m³.
Diameter estimate:
A_net = Q_V / (f·u_f), A_total = A_net / (1 − downcomer area fraction), D = √(4·A_total/π)
- Q_V – vapour volumetric flow, m³/s; f – design fraction of flooding (0.7–0.85).
Packed height from HETP:
Z = N_theoretical × HETP
Worked examples
Example 1 – actual trays and Murphree efficiency (standard). Given: a separation needs 12 theoretical stages in the column; overall efficiency 0.65. On one tray, vapour enters at y = 0.50, leaves at y = 0.60, and the vapour in equilibrium with the liquid leaving the tray would be y* = 0.70.
- N_actual = 12/0.65 = 18.46 → 19 trays (always round up).
- E_MV = (0.60 − 0.50)/(0.70 − 0.50) = 0.10/0.20 = 0.50.
19 actual trays; E_MV = 0.50 (50 %).
Example 2 – column diameter from flooding (GATE level). Given: vapour mass flow 3.0 kg/s, ρ_V = 2.5 kg/m³, ρ_L = 800 kg/m³, C_SB = 0.08 m/s; design at 80 % of flooding; downcomers take 12 % of the cross-section.
u_f = C_SB √((ρ_L − ρ_V)/ρ_V)= 0.08 × √(797.5/2.5) = 0.08 × 17.86 = 1.429 m/s.- Design velocity u = 0.80 × 1.429 = 1.143 m/s.
- Q_V = 3.0/2.5 = 1.20 m³/s; A_net = 1.20/1.143 = 1.050 m².
- A_total = 1.050/0.88 = 1.193 m².
- D = √(4 × 1.193/π) = 1.23 m.
Column diameter ≈ 1.23 m (a standard size such as 1.25 m would be chosen).
Common mistakes
- Dividing by efficiency and rounding down – actual trays are always rounded up, and a partial reboiler counts as a theoretical stage but not a tray.
- Writing Murphree efficiency with y* in equilibrium with the entering liquid instead of the liquid leaving the tray.
- Saying packed columns are easier to clean or better with solids – trays are.
- Designing at 100 % of flooding, or using total area instead of net area in the flooding velocity.
- Confusing loading point (holdup begins to rise) with flooding point (liquid can no longer flow down).
- Forgetting liquid redistributors in tall packed beds.
For GATE CH
Expect conceptual questions on tray types, weeping, entrainment, flooding, loading and flooding points, and tray-versus-packing selection; numericals on actual trays from overall efficiency, Murphree efficiency from compositions, and column diameter from a flooding velocity. Read pressure-drop and flooding data carefully when a chart value is given in the question.
Quick check
- Which tray type does not weep at low vapour rates, and why?
- Define overall tray efficiency.
- What happens at the loading point of a packed column?
- Name two situations where a packed column is preferred over a tray column.
Answers: 1. bubble-cap tray, because the cap and riser keep a liquid seal; 2. theoretical stages divided by actual trays; 3. liquid holdup begins to increase with gas rate and the pressure-drop line steepens; 4. any two of vacuum service, corrosive liquids, foaming systems, small diameters, heat-sensitive materials.
Interview questions
All Mass Transfer interview questionsTry answering each one aloud before you open it.
1.What is a tray column in the context of gas-liquid contact equipment?Concept
A tray column is a type of gas-liquid contactor used in chemical engineering processes. It consists of a vertical cylindrical shell with horizontal trays or plates inside. These trays facilitate the contact between the gas and liquid phases, allowing mass transfer to occur. The liquid flows across the trays, while the gas moves upwards through openings in the trays, promoting efficient mixing and mass transfer.
2.Explain the working principle of a packed column.Concept
A packed column is a type of gas-liquid contactor where the column is filled with packing material instead of trays. The packing provides a large surface area for the gas and liquid to come into contact. The liquid flows down the column over the packing surface, while the gas flows upwards. The interaction between the two phases on the packing surface facilitates mass transfer, making packed columns suitable for absorption, stripping, and distillation processes.
3.Why are bubble cap trays used in tray columns?Application
In a bubble-cap tray vapour rises through risers and is turned down under caps, leaving through slots into the liquid. Because the riser projects above the tray floor, a liquid seal is kept even at very low vapour rates, so bubble caps do not weep and have a very wide turndown. Their drawbacks are high cost, higher pressure drop and lower capacity than sieve or valve trays, so today they are used mainly where very low vapour or liquid rates must be handled.
4.What happens if the liquid flow rate in a packed column is too high?Application
If the liquid flow rate in a packed column is too high, it can lead to flooding. Flooding occurs when the liquid accumulates in the column, restricting the upward flow of gas. This results in a significant pressure drop and can severely reduce the efficiency of the mass transfer process. To prevent flooding, the flow rates must be carefully controlled and the column design must accommodate the expected operating conditions.
5.How does the choice of packing material affect the performance of a packed column?Application
The choice of packing material affects the performance of a packed column by influencing the surface area available for mass transfer, the pressure drop across the column, and the column's capacity. Different materials and shapes provide varying levels of efficiency and pressure drop. For example, structured packings offer high surface area and low pressure drop, making them suitable for high-efficiency separations, while random packings are often used for general applications due to their cost-effectiveness.
6.What is the purpose of downcomers in tray columns?Concept
Downcomers in tray columns are used to direct the flow of liquid from one tray to the next. They ensure that the liquid flows in a controlled manner, preventing it from bypassing the trays and ensuring proper contact with the rising gas. Downcomers help maintain the liquid level on each tray, which is crucial for efficient mass transfer and separation.
7.Explain the term 'weeping' in the context of tray columns.Concept
Weeping in tray columns occurs when the liquid leaks through the perforations in the trays instead of flowing across the tray surface. This can happen if the vapor flow rate is too low to support the liquid on the tray. Weeping reduces the contact area between the gas and liquid phases, leading to decreased mass transfer efficiency. It is important to maintain appropriate vapor and liquid flow rates to prevent weeping.
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