Design of trays and packed columns

In short

Tray and packed column design converts a separation duty - already fixed as number of theoretical stages or transfer units by distillation, absorption or extraction calculations - into real hardware: a shell of a definite diameter and height, with trays or packing inside, plus downcomers, weirs, distributors and supports. The designer sizes the diameter from vapour-handling (flooding) limits and the height from stage efficiency (tray spacing × actual trays) or HETP × theoretical stages, then checks pressure drop, weeping, entrainment, turndown and mechanical strength. It is examined in GATE Chemical Engineering (Mass Transfer section), university Mass Transfer-II and Process Equipment Design papers, and is the daily work of process and equipment design engineers in Indian EPC and operating companies - EIL, L&T Hydrocarbon, Toyo, Technip, Reliance, IOCL, BPCL, Tata Chemicals, UPL, Deepak Nitrite. Pressure vessel aspects follow IS 2825 / ASME Section VIII Div. 1, and operating plants are governed by the Occupational Safety, Health and Working Conditions Code, 2020 (in force 21 November 2025) and SMPV / Petroleum Rules for pressure and hydrocarbon service.

Explained

What the designer is given and what must be produced

• Given (from stage calculations): feed and product flows, compositions, temperatures, pressures; number of theoretical stages N or transfer units NTU; reflux or solvent rate; physical properties (densities, viscosities, surface tension, diffusivities).
• To be produced: column diameter D, type of internal (tray type or packing type and size), tray spacing or packed-bed depth, number of actual trays or HETP-based height, total tangent-to-tangent height, pressure drop, downcomer/distributor details, nozzle and support design, and material of construction.
• Two independent design directions: *diameter* is a hydraulic (capacity) question set by flooding; *height* is a mass-transfer (efficiency) question set by tray efficiency or HETP.
• Design basis flows: always size at the *worst* section - usually the tray with the largest vapour volumetric flow (often just above the feed in a stripping-heavy column, or the top in a rectifying-heavy one). Many columns are swaged (two diameters) when top and bottom loads differ by more than about 20%.

Tray types, construction and selection

• Sieve tray: flat plate perforated with 3-12 mm holes, hole area 5-15% of active area. Cheapest, easiest to fabricate and clean; turndown only about 2:1 to 3:1 because vapour velocity must stay high enough to prevent weeping.
• Valve (float valve) tray: holes fitted with liftable caps; the valve opens progressively with vapour rate, so turndown is 4:1 to 5:1 at nearly sieve-tray cost. The industry workhorse in Indian refineries today.
• Bubble-cap tray: riser with slotted cap dipping in liquid; liquid is sealed on the tray so it cannot weep - turndown 8:1 or better, but 2-3 times costlier and the highest pressure drop. Retained for very low liquid rates and wide-turndown service.
• High-capacity / proprietary trays: multi-downcomer, Superfrac, MVG, Nye trays - give 15-40% more capacity in revamps.
• Flow patterns: cross-flow (single-pass) for normal rates; two-pass and four-pass trays for high liquid loads in large columns (D > ~2 m); reverse-flow trays for very low liquid rates.
• Mechanical details: trays usually 3-5 mm thick in SS, 5-6 mm in CS; sectional removable panels for columns above ~0.9 m (man-entry); tray support rings and beams welded to the shell; manholes every 10-15 trays for maintenance.
• Key tray geometry terms: *column area Ac*, *downcomer area Ad* (6-15% of Ac each side), *active/bubbling area Aa = Ac − 2Ad* for a two-downcomer tray or *Ac − Ad − calming zones* for single-pass, *hole area Ah*, *weir length lw ≈ 0.6-0.85 D*, *weir height hw = 40-50 mm* (atmospheric), 20-25 mm (vacuum), up to 90 mm (high-pressure absorbers), *tray spacing 300-600 mm* (450-600 mm typical, 300-350 mm only in small or vacuum columns).

Tray hydraulics: the operating window

• Flooding: liquid cannot flow down, so it accumulates and is carried up. Two causes - *entrainment (jet) flooding*, vapour carrying liquid to the tray above, and *downcomer flooding/backup*, liquid level in the downcomer rising to the tray above. Design normally at 80-85% of flooding velocity.
• Weeping: liquid drains through the holes instead of the downcomer at low vapour rate; severe weeping is *dumping*. Checked against a minimum vapour velocity through holes (Fair's weep-point correlation).
• Entrainment: fractional liquid carryover ψ; keep ψ below about 0.1 so Colburn's efficiency correction stays small.
• Coning / vapour blowing: at very high vapour and very low liquid rate, vapour pushes liquid aside and bypasses contact.
• Downcomer backup: hdc = hw + how + ht + hda must remain below tray spacing (typically ≤ half the spacing plus weir height in clear-liquid terms); downcomer residence time ≥ 3 s (5 s for foaming systems) for vapour disengagement.
• Tray pressure drop: ht = hd (dry hole drop) + hl (clear liquid, ≈ hw + how) + hσ (surface tension/residual). Typically 50-80 mm liquid per tray, i.e. about 0.3-0.7 kPa per tray, so roughly 10-20 kPa across a 30-tray atmospheric column.
• Turndown ratio: maximum to minimum stable vapour throughput - the band between flooding and weeping.
• Foaming: amine, caustic and glycol systems need a *system factor* (0.5-0.85) applied to capacity, wider tray spacing and larger downcomers.

Packing types, internals and selection

• Random (dumped) packing: Raschig rings (obsolete), Pall rings, Intalox saddles, metal/plastic/ceramic high-performance rings (IMTP, Nutter ring, Cascade Mini Ring). Sizes 16-90 mm; size ≤ D/8 so wall effects stay small. Cheap, fouling-tolerant, random orientation.
• Structured (regular) packing: corrugated sheet metal or gauze (Mellapak, Flexipac, Sulzer BX) with specific surfaces 125-750 m²/m³. Very low pressure drop per theoretical stage (0.3-1.0 mbar) and low HETP - the standard choice for vacuum distillation and heat-sensitive duties.
• Grid packing: open steel grids for fouling, coking or heat-transfer sections (refinery vacuum wash zones).
• Bed internals: liquid distributor (orifice-pan, trough, spray - 40-100 drip points per m², critical to performance), *hold-down plate* (metal packing) or bed limiter, *support plate* (gas-injection type so that flooding does not start at the support), *redistributor* every 5-10 column diameters or 6-8 m of bed to correct wall flow and maldistribution, and *collector trays* for side draws.
• Characterising numbers: specific surface a (m²/m³), void fraction ε, packing factor Fp (1/ft or 1/m) used in flooding charts, HETP.
• Why maldistribution matters: poor initial distribution or wall flow destroys the apparent efficiency; HETP can double. Distributor quality is usually the real cause when a packed column "under-performs".

Trays versus packing - how the choice is made

• Prefer packing: low pressure drop needed (vacuum distillation, large stage counts), corrosive service (ceramic/plastic), foaming systems, small diameters (< 0.6 m, where trays are hard to fabricate and inspect), heat-sensitive products, revamps needing extra capacity in an existing shell, absorption with low liquid rates.
• Prefer trays: solids or fouling duty, large liquid holdup wanted for reaction or slow mass transfer, wide turndown, many side draws and interstage coolers, very high liquid loads, very large diameters (packing cost and distribution risk rise), where reliable scale-up from published efficiencies matters.
• Cost rule of thumb: trays cheaper per unit volume at large diameter; structured packing is costly but recovers it through lower column height and energy (smaller pressure drop → lower reboiler temperature).

Height from efficiency: trays

• Overall column efficiency Eo = N_theoretical / N_actual; typical 0.5-0.7 for distillation, 0.1-0.5 for absorption of sparingly soluble gases.
• Murphree vapour efficiency EmV for one tray, based on the vapour leaving against the vapour in equilibrium with the leaving liquid; *point efficiency Eog* is local, and EmV > Eog when liquid is unmixed across the tray.
• O'Connell correlation gives Eo from (relative volatility × liquid viscosity) for distillation, and from Henry's constant, pressure and viscosity for absorption - the standard exam correlation.
• AIChE / Chan-Fair methods: break the tray into gas-phase and liquid-phase transfer units, plus a mixing and an entrainment correction; used professionally, outlined in exams.
• Apparent efficiency includes entrainment: Ea = EmV / [1 + EmV·ψ/(1−ψ)].
• Height: Z = (N_actual − 1) × tray spacing + top vapour space (~1-1.5 m) + bottom sump (5-10 min liquid holdup) + skirt.

Height from transfer units: packing

• Z = HETP × N_theoretical (equilibrium-stage route) or Z = HTU × NTU (transfer-unit route, more fundamental for absorption).
• HOG = G/(KG·a·S) and HOL = L/(KL·a·S); Z = HOG × NOG.
• Rule of thumb: HETP ≈ packing nominal size in inches × 1 ft is crude; modern values are 0.3-0.6 m for 25-50 mm random packings and 0.2-0.5 m for structured packing. Vendor data governs.
• Correlations: Onda for kG, kL and wetted area; Billet-Schultes and SRP (Rocha-Bravo-Fair) models for HETP and pressure drop; Eckert generalised pressure-drop correlation (GPDC) chart for flooding and ΔP.
• Minimum wetting rate: about 0.5-2 m³/(m²·h) for random packing - lowest for ceramics, which wet readily (≈0.5), and highest for bright metal and plastic (up to ~2); below it the packing dries out and efficiency collapses.
• Design pressure drop: 15-50 mm water per metre of packing for absorbers and strippers, 40-80 mm/m for atmospheric and moderate-pressure distillation, 8-25 mm/m for vacuum.

Codes, standards and Indian practice

• Pressure shell: designed to IS 2825 (code for unfired pressure vessels) or ASME Boiler & Pressure Vessel Code Section VIII Div. 1 - the latter is what most Indian EPC jobs specify; nozzles, flanges and gaskets to ASME B16.5/B16.47.
• Wind and seismic loading on tall columns from IS 875 (Part 3) and IS 1893; foundation and skirt from IS 456 / IS 800. Tall columns are checked for combined axial, wind-bending and seismic stresses and for elastic stability (shell thickness often set by wind, not pressure).
• Insulation, ladders, platforms, trays per OISD standards (OISD-STD-118, -116 etc.) in oil and gas; ladders/platforms and confined-space entry under the Occupational Safety, Health and Working Conditions Code, 2020 (in force from 21 November 2025, replacing the Factories Act, 1948).
• Relief: every column needs over-pressure protection sized per API 520/521; flare and vent routing per OISD and the Petroleum and Natural Gas Rules. Environmental clearance and emission limits from CPCB/SPCB under the Environment (Protection) Act, 1986.
• Hazardous-area classification for the column area as per IS/IEC 60079 series; hydrocarbon columns are usually Zone 1/Zone 2.
• Exact clause numbers and editions of Indian Standards are revised periodically - confirm the current edition of IS 2825, IS 875 (Part 3) and IS 1893 before using them in a real design.

Key points

• Diameter - set by flooding (capacity), designed at 80-85% of flooding velocity, at the section with the highest vapour load.
• Height - set by mass transfer: (N_actual − 1) × tray spacing for trays, HETP × N or HTU × NTU for packing.
• Souders-Brown equation - un = C√[(ρL − ρV)/ρV]; C from Fair's chart against flow parameter FLV and tray spacing, corrected for surface tension and foaming.
• Flow parameter FLV = (L/V)√(ρV/ρL) - the single variable that places a tray on Fair's capacity chart; high FLV means downcomer-limited, low FLV entrainment-limited.
• Weeping - low-vapour-rate limit; flooding - high-vapour-rate limit; the gap between them is turndown.
• Turndown - sieve 2-3:1, valve 4-5:1, bubble cap 8:1 or more.
• Downcomer - 6-15% of column area per downcomer, residence time ≥ 3 s (5 s foaming), backup < tray spacing.
• Weir height 40-50 mm normal, 20-25 mm vacuum; weir length 0.6-0.85 D; hole area 5-15% of active area.
• Tray pressure drop ht = hd + hl + hσ, typically 50-80 mm liquid per tray.
• Packing factor Fp - the single number characterising a packing's resistance; lower Fp means higher capacity.
• HETP - height of packing equivalent to one theoretical plate; 0.3-0.6 m for random, 0.2-0.5 m for structured packing.
• Minimum wetting rate - below about 0.5-2 m³/(m²·h) random packing dries and efficiency collapses.
• Liquid distribution - the commonest cause of poor packed-column performance; redistribute every 5-10 diameters.
• O'Connell correlation - overall tray efficiency from α·μL for distillation; typical Eo 0.5-0.7.
• Murphree efficiency - per-tray efficiency; apparent efficiency corrects it for entrainment.
• Trays for fouling, solids, high liquid loads, wide turndown and side draws; packing for vacuum, corrosion, foaming, small diameters and revamps.
• Codes - IS 2825 or ASME Section VIII Div. 1 for the shell; IS 875 Part 3 and IS 1893 for wind and seismic; relief per API 520/521.

Formulas and examples

Example 1 - Tray column diameter (Fair method). A distillation column top section: vapour 9,000 kg/h, ρ_V = 2.4 kg/m³; liquid 7,200 kg/h, ρ_L = 760 kg/m³; σ = 20 mN/m; tray spacing 450 mm; non-foaming.

1. V_w = 9,000/3,600 = 2.5 kg/s; L_w = 7,200/3,600 = 2.0 kg/s.
2. F_LV = (2.0/2.5)·√(2.4/760) = 0.8 × √0.003158 = 0.8 × 0.0562 = 0.0450.
3. From Fair's chart at F_LV = 0.045, 450 mm spacing: C_sb ≈ 0.090 m/s. F_ST = (20/20)^0.2 = 1, F_F = 1, F_HA = 1.
4. u_nf = 0.090 × √[(760 − 2.4)/2.4] = 0.090 × √315.7 = 0.090 × 17.77 = 1.60 m/s.
5. Design at 82%: u_n = 1.31 m/s. Vapour volumetric flow V̇ = 2.5/2.4 = 1.042 m³/s.
6. A_net = 1.042/1.31 = 0.795 m². Take A_d = 12% of A_c → A_net = 0.88 A_c → A_c = 0.795/0.88 = 0.903 m².
7. D = √(4 × 0.903/π) = √1.150 = 1.072 m → specify 1.1 m (standard shell size).

Example 2 - Tray pressure drop and downcomer backup. Same column: D = 1.1 m → A_c = 0.950 m²; A_d = 0.114 m² (12%); A_a = A_c − 2A_d = 0.722 m²; hole area A_h = 10% of A_a = 0.0722 m²; weir length l_w = 0.77 × 1.1 = 0.85 m; h_w = 45 mm; C_o = 0.84; apron clearance 25 mm → A_ap = 0.85 × 0.025 = 0.0213 m².

1. Hole velocity u_h = V̇/A_h = 1.042/0.0722 = 14.43 m/s.
2. h_d = 51 × (14.43/0.84)² × (2.4/760) = 51 × (17.18)² × 0.003158 = 51 × 295.2 × 0.003158 = 47.6 mm liquid.
3. h_ow = 750 × [2.0/(760 × 0.85)]^2/3 = 750 × (0.003096)^2/3. (0.003096)^2/3 = exp(0.6667 × ln 0.003096) = exp(0.6667 × (−5.777)) = exp(−3.851) = 0.02125 → h_ow = 15.9 mm.
4. Residual drop h_σ ≈ 12.5×10³/ρ_L = 12,500/760 ≈ 16.4 mm (take 16 mm). Total h_t = 47.6 + (45 + 15.9) + 16 = 124.5 mm liquid.
5. ΔP per tray = 9.81×10⁻³ × 124.5 × 760 = 928 Pa ≈ 0.93 kPa. For 30 trays ≈ 28 kPa.
6. h_dc = 166 × [2.0/(760 × 0.0213)]² = 166 × (0.1236)² = 166 × 0.01528 = 2.5 mm.
7. Backup h_b = (45 + 15.9) + 124.5 + 2.5 = 187.9 mm < tray spacing + h_w = 450 + 45 = 495 mm → safe.
8. Downcomer residence time t_r = A_d·h_b·ρ_L/L_v = 0.114 × 0.1879 × 760/2.0 = 8.14 s > 3 s → satisfactory.

Example 3 - Actual trays and column height. A column needs 22 theoretical stages (excluding reboiler). Average relative volatility α = 2.2, average liquid viscosity μ_L = 0.25 cP.

1. α·μ_L = 2.2 × 0.25 = 0.55. E_o ≈ 0.492 × (0.55)^−0.245.
2. ln 0.55 = −0.5978; ×(−0.245) = 0.1465; exp(0.1465) = 1.158 → E_o = 0.492 × 1.158 = 0.570.
3. N_actual = 22/0.570 = 38.6 → 39 trays (plus the reboiler as one stage already excluded).
4. Height of trayed section = (39 − 1) × 0.45 = 17.1 m.
5. Add 1.2 m top vapour space, 2.0 m bottom sump, 1.5 m skirt → tangent-to-tangent ≈ 20.3 m, overall ≈ 21.8 m. Such a column (L/D ≈ 18) needs a wind and seismic check per IS 875 Part 3 and IS 1893.

Example 4 - Packed height by HETP and by HTU-NTU. Same 22 theoretical stages in 50 mm metal Pall rings, HETP = 0.55 m, with redistribution every 6 m.

1. Z_packing = 22 × 0.55 = 12.1 m → three beds of about 4 m each, two redistributors.
2. ΔP at 25 mm water per metre of packing = 12.1 × 25 = 302 mm water ≈ 2.96 kPa total - against 28 kPa for 30 trays in Example 2, which is why vacuum towers use packing.
3. Transfer-unit route for an absorber: G_m = 0.030 kmol/(m²·s), K_G·a = 0.040 kmol/(m³·s·bar) at 1 bar → H_OG = 0.030/0.040 = 0.75 m. For N_OG = 6, Z = 0.75 × 6 = 4.5 m; with 15% design margin, specify 5.2 m of packing.

Example 5 - Turndown check (sieve vs valve tray). Design hole velocity in Example 2 is 14.43 m/s. Weep point for h_w = 45 mm, h_ow = 15.9 mm requires h_d + h_σ ≥ 60.9 mm, and with h_σ = 16 mm → h_d ≥ 44.9 mm.

1. h_d = 51·(u_h/0.84)²·0.003158 = 0.2282·u_h² (mm, with u_h in m/s). Setting 0.2282·u_h² = 44.9 → u_h² = 196.8 → u_h(min) = 14.03 m/s.
2. Design u_h = 14.43 m/s clears the weep point by only about 3% - the tray is stable at design but will weep as soon as the column is turned down at all, so there is effectively no turndown. Fix: cut hole area from 10% to 8% of A_a (A_h = 0.0578 m² → u_h = 18.0 m/s, h_d = 74 mm, comfortably above the weep point) or switch to valve trays for 4-5:1 turndown.
3. This is exactly how a hydraulic check feeds back into tray geometry - diameter, hole area, weir height and tray type are iterated, not chosen once.

Where it is used

• Refineries and petrochemicals: crude and vacuum distillation units, FCC main fractionators, naphtha splitters, amine absorbers and strippers at IOCL, BPCL, HPCL, Reliance Jamnagar, HMEL, NRL - valve trays, grid and structured packing in vacuum sections.
• EPC and design consultancies: process and equipment design engineers at EIL, L&T Hydrocarbon, Toyo India, Technip Energies, Jacobs, Tata Consulting Engineers prepare datasheets and hand hydraulics to internals vendors (Sulzer, Koch-Glitsch, RVT, Chemiequip, Finepac India).
• Bulk drugs, agrochemicals and specialty chemicals: solvent recovery and distillation columns at Divi's, Aurobindo, UPL, Deepak Nitrite, SRF, Atul - packed glass-lined or SS columns for corrosive and small-volume duties.
• Fertilisers, gas processing and ASUs: CO₂ absorbers and urea strippers at IFFCO, RCF, Chambal; structured packing in cryogenic air separation columns at Linde and INOX Air Products, where ΔP directly drives compressor power.
• Operations, revamps and troubleshooting: plant engineers run gamma scans and ΔP surveys to find flooding, weeping or maldistribution, and revamp with high-capacity trays or structured packing to debottleneck without a new shell.
• Water, environment and food: stripping columns for ammonia and VOC removal, deaerators, and ethanol distillation in sugar and grain distilleries under the Ethanol Blended Petrol programme.

Interview questions

• Q: What fixes the diameter of a distillation column and what fixes its height? A: Diameter is fixed by hydraulic capacity - the flooding vapour velocity, with design at 80-85% of flooding; height is fixed by mass transfer - actual trays × tray spacing, or HETP × theoretical stages.
• Q: What is the flow parameter F_LV and why is it used? A: F_LV = (L/V)√(ρ_V/ρ_L); it is the x-axis of Fair's capacity chart and indicates whether the tray is limited by entrainment (low F_LV, vapour-loaded) or by the downcomer (high F_LV, liquid-loaded).
• Q: Differentiate weeping, entrainment and flooding. A: Weeping is liquid draining through tray holes at low vapour rate; entrainment is liquid droplets carried up by vapour; flooding is accumulation of liquid in the column from excessive entrainment or downcomer backup, and it ends useful operation.
• Q: Why do bubble-cap trays give the best turndown? A: The cap and riser keep a liquid seal on the tray regardless of vapour rate, so the tray cannot weep even at very low vapour flow - at the cost of the highest pressure drop and cost.
• Q: When would you choose packing over trays? A: Vacuum or low-ΔP service, corrosive fluids, foaming systems, heat-sensitive products, columns below about 0.6 m diameter, and revamps needing more capacity or stages in an existing shell.
• Q: What is HETP and what does it depend on? A: The depth of packing that does the job of one theoretical plate; it depends on packing type and size, liquid and vapour loads, physical properties and - very strongly - on liquid distribution quality.
• Q: Why does a packed column need a redistributor? A: Liquid drifts to the wall and channels as it flows down, so after roughly 5-10 column diameters or 6-8 m the liquid is collected and redistributed to restore uniform wetting and efficiency.
• Q: A packed absorber is giving far fewer stages than designed. What do you check first? A: Liquid distribution and wetting - a plugged or tilted distributor, liquid rate below the minimum wetting rate, or a damaged bed - before blaming the packing or the mass-transfer correlation.
• Q: Why design at 80-85% of flooding rather than 95%? A: Flooding correlations carry ±15-20% uncertainty, and plants are run above design; the margin keeps the column stable through feed-rate upsets, fouling and property changes.
• Q: Why is downcomer residence time specified? A: The downcomer must let entrained vapour escape from the liquid; at least 3 s (5 s for foaming systems) is needed, otherwise aerated liquid backs up and the column floods prematurely.
• Q: What is the difference between Murphree and overall column efficiency? A: Murphree efficiency compares the actual composition change on one tray with the equilibrium change on that tray; overall efficiency is the ratio of theoretical to actual trays for the whole column and is what sets tray count.
• Q: Which code would you use for the shell of a 25 m tall atmospheric distillation column in India, and what usually sets its thickness? A: IS 2825 or ASME Section VIII Div. 1 for the pressure shell, with wind loading from IS 875 (Part 3) and seismic from IS 1893 - for a tall, low-pressure column the thickness is usually set by wind-induced bending and shell stability, not internal pressure.
• Q: How do you size a two-pass tray and when is it needed? A: When liquid load per unit weir length exceeds roughly 50-60 m³/(h·m) - usually in large-diameter, high-liquid-rate columns; liquid is split into two paths with a central and two side downcomers, halving weir load and downcomer duty.

Common mistakes

• Using the total column area instead of the net area (or active area) in flooding and velocity calculations, which oversizes or undersizes the diameter.
• Sizing the column at the average vapour load instead of the maximum-load section - top and bottom loads can differ by 50% around the feed.
• Mixing up clear-liquid heights (mm liquid) with pressure (Pa or mm water); every term in the tray drop equation must be in the same liquid height basis before conversion.
• Checking only flooding and forgetting the weeping and turndown check, so the tray weeps at part load - as in Example 5.
• Treating HETP as a property of the packing alone; it changes with load, properties and especially distribution quality, and vendor data plus a margin is mandatory.
• Forgetting that adding trays raises pressure drop, which raises the bottom temperature and can thermally damage the product - the reason vacuum columns are packed.
• Quoting withdrawn or superseded IS numbers in a design report, or assuming the old Factories Act, 1948 still applies - the Occupational Safety, Health and Working Conditions Code, 2020 has governed since 21 November 2025.
• Omitting the reboiler as an equilibrium stage, or dividing it by tray efficiency, when converting theoretical to actual trays.

Related topics

Distillation · Absorption and stripping · Mass transfer: diffusion · Fluid mechanics: flow through pipes and packed beds · Process equipment design and pressure vessels · Extraction and leaching