Process Columns: Trays vs Packing, Internals, Design and Fabrication

Quick Answer

A process column is a pressure vessel with internals that bring two phases into repeated contact: vapour and liquid for distillation, absorption and stripping, or two liquids for extraction. The internals are trays or packing. Trays (sieve, valve or bubble-cap) suit large diameters, high liquid rates, high pressure, fouling service and side draws, and are the easiest internals to inspect; 600 mm is the usual tray spacing. Packing, random rings and saddles or structured corrugated sheets, gives far lower pressure drop: around 0.3–1 mbar per theoretical stage for structured packing, against roughly 5–8 mbar per actual tray at atmospheric pressure, which is why vacuum columns are packed. Structured packing also needs less height per stage, with planning HETPs of about 0.25–0.45 m against the 0.8–1.0 m a 600 mm tray delivers at 60–75% efficiency. A packed column is only as good as its liquid distribution: beds are normally limited to about 20 theoretical stages, and distributors keep flow variation within about 5%. Mechanically, tall columns are governed by wind, earthquake, erection and test loads as well as pressure, and should be designed around their transport limits from the start.

A data sheet that says "30 theoretical stages" has decided the separation, not the column. As trays, those stages might be 46 trays in a 31 m shell; as structured packing, 12 m of packing in an 18 m shell with about a seventh of the pressure drop. The choice sets the diameter, the bottoms temperature, the foundation load, the inspection regime and whether the column can travel in one piece.

This guide covers how that choice is made, the internals that decide performance, and the mechanical points to settle before the drawing is frozen.

What a Process Column Does

Columns separate by repeated contact between two phases:

• Distillation — reboiler vapour rises against reflux, concentrating the light components at the top

• Absorption — a solvent removes a component from a gas, as in amine absorbers and caustic scrubbers

• Stripping — steam, inert gas or reboiled vapour removes a dissolved component from a liquid, as in sour water strippers and amine regenerators

• Liquid–liquid extraction — a solvent removes a component from another liquid

Each column is two designs in one. The internals do the separation and set the diameter and height. The vessel contains the pressure and carries the internals, platforms and insulation against wind, earthquake and erection loads. The two are often designed by different companies, and the costly mistakes happen at the interface.

Trays

A tray holds a layer of liquid that vapour bubbles through. Liquid crosses the tray, flows over an outlet weir and down a downcomer to the tray below.

Tray types

Tray hydraulics in brief

• Flooding. Too much vapour carries liquid up to the tray above (jet flood); liquid backs up the downcomer to the tray above (downcomer flood), from high liquid rates, high tray pressure drop or a restricted downcomer. Columns are normally designed at about 75–85% of flood.

• Weeping. Too little vapour lets liquid drain through the deck openings, bypassing the contact. The ratio between the flooding and weeping limits is the turndown.

• Downcomers. Liquid needs typically 3–7 seconds in the downcomer to release its vapour, longer if it foams, and clear-liquid backup is kept below half of the sum of tray spacing and weir height.

• Weir height, commonly 25–50 mm, sets the liquid depth and so the contact time and pressure drop.

Tray spacing

The default is 600 mm (24 in): it balances height against capacity and leaves room to move between trays for inspection. About 450–760 mm is normal. Below 450 mm inspection becomes difficult, and vacuum columns often use 600–760 mm to limit entrainment.

Tray efficiency

Overall efficiency, theoretical stages divided by actual trays, turns a simulation into a column. A long-standing first estimate is the O'Connell correlation, E₀ = 50.3 × (α × μ)^−0.226 in percent, with α the relative volatility of the key components and μ the liquid viscosity in cP at the average column temperature. For α = 2.0 and μ = 0.15 cP it gives about 66%. Without plant data, ±10% is the realistic accuracy; operating data from a similar column is the best basis. Absorbers need separate methods, as their efficiencies are often far lower.

Packing

Packing replaces trays with a continuous bed: liquid spreads over it as a film while vapour rises through the voids.

Random packing

Rings and saddles are dumped into the column, from Raschig rings to slotted Pall rings and modern high-performance shapes, in sizes of about 13–90 mm. Larger sizes give capacity and low pressure drop; smaller sizes give efficiency.

• Metal is the strongest, wets well and is the usual choice in distillation.

• Ceramic resists hot acids but is brittle.

• Plastic is cheap and corrosion resistant, and is used mainly in absorption, scrubbing and stripping. Limits are about 104°C for general-grade polypropylene, 143°C for PVDF and 250°C for PFA, lower in deep beds and at high liquid loads. Plastic can also wet poorly at low liquid rates.

Structured packing

Structured packing is corrugated sheet, usually metal, stacked in blocks with the corrugations crossing, commonly at 45° from the horizontal; a steeper 60° gives more capacity and lower pressure drop at some cost in efficiency. Specific surface areas run from about 55 to 725 m²/m³, and more area means more efficiency and less capacity. Wire gauze packings give the most stages per metre and wet well at very low liquid rates, which suits deep vacuum. Heavy grid types handle severe fouling and coking.

HETP and what it really means

Packed height is usually estimated as theoretical stages × HETP, the height equivalent to a theoretical plate. As planning figures, common sheet-metal structured packings give roughly 0.25–0.45 m and modern random packings 0.45–0.9 m. Real HETP varies with the system, loads and above all distribution quality, so the supplier's tested values govern; for absorption and stripping, transfer units are the better basis.

Distribution decides packed-column performance

A packed bed cannot correct bad distribution. Uneven liquid gives each part of the bed a different liquid-to-vapour ratio, so the bed performs as if it had fewer stages; in large-scale tests, zonal maldistribution cut random packing efficiency by about 20%. The rules:

• Flow variation between drip points within about 5%, and about 3% at design rate for high-performance distributors

• Drip point density of roughly 40–130 per m², at the high end for high-surface-area packings

• Head and turndown: orifice flow varies with the square root of the liquid head, so a 4:1 turndown needs 16 times the head. A distributor with 50 mm of head at minimum flow needs about 800 mm at maximum, which is why wide turndown calls for multi-level orifices or tall distributors.

• Bed height: redistribute about every 20 theoretical stages or 9–10 m, whichever comes first, and wherever a feed enters or a side draw leaves

Trays vs Packing: Selection Table

Internals That Make or Break Performance

• Liquid distributors — trough, pan, deck, pipe-arm or spray types, chosen by diameter, liquid rate, turndown and fouling. Orifice types distribute most evenly but foul most easily; weir and V-notch types tolerate dirt but distribute less evenly.

• Collectors and chimney trays — collect liquid for draw-offs, pumparounds or redistribution while risers pass the vapour.

• Support plates — carry the bed without limiting capacity: gas-injection supports for random packing, open grids for structured.

• Hold-down plates and bed limiters — stop random packing lifting in an upset.

• Feed devices — a flashing or two-phase feed needs a gallery, vapour horn or inlet diffuser, or it scatters liquid and jets vapour into the bed.

• Mist eliminators — mesh pads or vane packs at the top outlet.

Worked Example: 30 Theoretical Stages, Two Ways

A vacuum distillation needs 30 theoretical stages, excluding the reboiler, at a top pressure of 100 mbar absolute, with a relative volatility of 2.0 and a liquid viscosity of 0.15 cP. The figures are illustrative; real designs use the internals supplier's rated hydraulics.

The trayed column is 13 m taller and its bottom pressure more than twice as high. Under vacuum that decides it: higher bottom pressure means a hotter reboiler, which degrades heat-sensitive products. At moderate and high pressure the gap narrows, and trays often win on cost, high liquid rates, fouling tolerance and inspection.

Mechanical Design and Fabrication

Shell, heads and loads

The shell and heads are sized for internal pressure, external pressure if the column runs under vacuum (usually with stiffening rings), and the combined longitudinal stresses from weight, wind and earthquake. On tall columns the lower courses are often thicker than pressure alone would need, because the bending moments peak at the base. Heads are chosen as for any pressure vessel (see our guide to pressure vessel heads), and nozzles reinforced as for heat exchanger shells and nozzles.

Tray support rings and levelness

Trays bolt or clamp to support rings, typically 40–90 mm wide, welded to the shell. An out-of-level tray runs with uneven liquid depth, costing efficiency and capacity. A common inspection limit is 3 mm out of level for columns up to 2 m in diameter and 6 mm above that. The rings must be level before the trays go in: correcting a ring after heat treatment and testing is a repair, not an adjustment. Tray manways, typically around 400 × 500 mm, let people pass between trays.

Skirt, base ring and elevation

Columns stand on a skirt welded to the bottom head, with a base ring and anchor bolt chairs. The skirt height is set by the bottoms pump's NPSH and the reboiler's circulation head. Skirts in hydrocarbon service are usually fireproofed, and hot columns need a skirt-to-head junction designed for the temperature gradient.

Wind, earthquake and vortex shedding

Wind and seismic loads set the base moment, the anchor bolts and often the bottom shell thickness. Slender columns also face vortex shedding: vortices shed at a frequency of about 0.2 × wind speed ÷ diameter, and if that matches the column's natural frequency, the column oscillates across the wind. For an insulated diameter of 1.6 m and a natural frequency of 0.7 Hz, the critical wind speed is about 0.7 × 1.6 ÷ 0.2 ≈ 5.6 m/s, an everyday breeze. The usual remedies are helical strakes on the upper part of the column, added damping or a stiffer design.

Attachments, lifting and erection

All clips for platforms, ladders, davits, insulation, pipe supports and fireproofing should be welded before post-weld heat treatment and the pressure test; welding to the shell afterwards means repeating the pressure test, and the heat treatment where one was required. Tall columns are lifted from horizontal to vertical on trunnions or lugs near the top and a tailing lug at the base, and the lift is a design load case: the column bends as it is raised.

Materials and cladding

Carbon steel covers most hydrocarbon service, stainless steel corrosive and cold duties, and clad plate or weld overlay on carbon steel is cheaper where only the inside surface needs alloy (see our guide to weld overlay and clad construction). Nickel-copper alloys such as Monel serve in hydrofluoric acid and similar duties. Low-temperature service needs materials impact tested down to the minimum design metal temperature.

Pressure testing: position matters

Tall columns are usually hydrotested horizontally in the shop. The test pressure must be set so every part reaches its required test pressure, including parts whose design pressure includes the liquid head of the vertical position, and the column and saddles must carry the water. Tested vertically on site, the water head adds to the pressure at the bottom and the water weight lands on the foundation. For a 3.0 m diameter column, 31.1 m tangent to tangent, with 2:1 ellipsoidal heads:

• Test water: about 227 m³, or about 227 t

• Shell and heads at a 20 mm wall: about 49 t, so the water weighs more than four times as much as the steel

• Static head when vertical: about 32.6 m of water, adding about 3.2 bar at the bottom

Check the bottom head, skirt and foundation for that case before deciding to test on site.

Transport and site assembly

Set the transport limits at the start of the design: routes, bridges and cranes all limit what can travel. The columns we supply ship as a whole unit up to 5 m in diameter and 30 m long, and in sections above that. Site joints need qualified procedures and welders (see our guide to WPS, PQR and welder qualification), radiography or ultrasonic testing, local heat treatment where required, and a site pressure test. Packing is loaded after erection, because transport and lifting would crush or displace it.

What to Send the Fabricator

• Process data sheet — service, fluids, operating and design conditions, minimum design metal temperature, corrosion allowance

• Internals specification — tray or packing type, number of trays or bed heights, hydraulic rating at minimum and maximum loads, distributor and feed device requirements

• Nozzle schedule — process nozzles, manways, instruments and draw-offs

• Site data — design wind speed, seismic zone, ambient temperature range

• Attachments — platforms, ladders, insulation and fireproofing

• Code and inspection — design code, inspection and test plan, NDT, heat treatment and certificate type (see our guide to EN 10204 material certificates)

• Logistics — transport route and site lifting constraints

Common Mistakes

1. Choosing internals from the design case alone. The column spends years at turndown or above design.

   Prevention: Rate the trays or packing at the minimum and maximum loads, and check the distributor head at both ends.

2. Taking a catalogue HETP as a design value. Real HETP depends on the system, the loads and the distribution.

   Prevention: Use the supplier's tested HETP for comparable service with an agreed margin, and transfer units for absorption and stripping.

3. Good packing, poor distributor. Maldistribution costs more stages than the choice of packing.

   Prevention: Specify drip point density, flow variation and levelness, and inspect the distributor at installation.

4. Structured packing in fouling service. Fine channels plug and the bed loses capacity.

   Prevention: Use trays with large openings, fixed valves or grid-type packing where solids or polymers are expected.

5. Welding clips after heat treatment or the pressure test. The test, and any required heat treatment, have to be repeated.

   Prevention: Freeze all clip locations before fabrication and weld them in the shop.

6. Testing a tall column vertically on site without checking the loads. The bottom head is overpressured and the foundation overloaded.

   Prevention: Check the bottom for the water head and the foundation and skirt for the water weight, or test horizontally in the shop.

7. Ignoring vortex shedding on a slender column. Cross-wind oscillation and fatigue cracking.

   Prevention: Check the critical wind speed against the natural frequency during design, and add strakes or damping where needed.

Supply from Kasko Makine

Industrial process columns

We design and manufacture high-performance columns specifically engineered for extreme environmental challenges, from Siberian freezing temperatures to processes at 400°C and higher.

• Scope: packing supports, distributors and shells prepared for installing the internals, plus operating bridges, maintenance platforms, ladders and davits

• Size and transport: columns up to 5 m in diameter and 30 m long are supplied as a whole unit; larger columns are supplied in parts for on-site assembly

• Materials: carbon steel, stainless steel, clad steel and special materials such as Monel

• Design codes and directives: ČSN, ASME, DIN, SEVT, EN and the Pressure Equipment Directive (PED)

Related equipment: pressure vessels, reactors, heat exchangers and equipment for pressure testing.

Engineering support: Send the process data sheet, the internals specification, the nozzle schedule, site wind and seismic data and any transport constraints. We will review the requirements and quote.

Send your column data sheet for a review and quotation — we reply within 24 hours. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading

• Heat Exchanger Manufacturing Process: How a Shell-and-Tube Unit Is Built, Step by Step

• Pressure Vessel Heads: 2:1 Ellipsoidal vs Torispherical vs Hemispherical

• Weld Overlay and Clad Construction

• Chemical Factory Setup and Plant Design Guide

Frequently Asked Questions

Q: What is the difference between a tray column and a packed column?
A: A tray column contacts vapour and liquid on a series of horizontal trays, with liquid crossing each tray and flowing down through downcomers. A packed column spreads the liquid as a film over random or structured packing while vapour rises through the voids. Trays tolerate fouling, high liquid rates and large diameters; packing gives lower pressure drop and less height per stage.

Q: When should structured packing be used instead of trays?
A: Structured packing is the usual choice for vacuum distillation, where a pressure drop of roughly 0.3 to 1 mbar per theoretical stage keeps the bottom pressure and temperature low, and for revamps that need more stages in an existing shell. Trays are often preferred at high pressure, at high liquid rates and in fouling or solids service.

Q: What is the typical tray spacing in a distillation column?
A: The usual default is 600 mm (24 in), which balances column height against capacity and leaves room for inspection between trays. Spacings between about 450 and 760 mm are normal. Below 450 mm it becomes difficult to move between trays for inspection, while vacuum and foaming services often use 600 to 760 mm to limit entrainment.

Q: What is HETP in a packed column?
A: HETP, the height equivalent to a theoretical plate, is the packed height that does the work of one theoretical stage. Common sheet-metal structured packings give roughly 0.25 to 0.45 m and modern random packings 0.45 to 0.9 m, but real values depend on the system, the loads and the liquid distribution, so the supplier's tested data governs the design.

Q: Why is liquid distribution so important in packed columns?
A: A packed bed cannot correct poor distribution. Uneven liquid gives parts of the bed different liquid-to-vapour ratios, so the bed behaves as if it had fewer stages; large-scale tests have shown zonal maldistribution cutting random packing efficiency by about 20%. Good distributors keep flow variation within about 5%, and beds are redistributed about every 20 theoretical stages.

Q: How is a tall column pressure tested?
A: Tall columns are usually hydrotested horizontally in the shop. The test pressure must be set so every part reaches its required test pressure, including parts designed for liquid head in the vertical position, and the supports must carry the water. A vertical site test adds about 0.1 bar of water head per metre of height at the bottom, about 3.2 bar for a column 31 m tangent to tangent, and loads the foundation.

Q: Can large process columns be transported in one piece?
A: It depends on the route, the bridges and the cranes, so transport limits should be fixed at the start of design. Kasko Makine supplies columns up to 5 m in diameter and 30 m long as a whole unit, and larger columns in parts for on-site assembly, which then needs qualified site welding, inspection of the closing seams and a site pressure test.