Packed-Bed Scrubber Sizing — and how the sizing actually works
This page does two things at once. It is a working preliminary sizing calculator for countercurrent wet packed-bed scrubbers — column diameter from hydraulics, packed height from mass transfer, liquid circulation, reagent consumption, pressure drop, power and operating cost. It is also a teaching page: every number it prints is produced by an equation that is displayed on the page, with your own numbers substituted into it, so you can follow the arithmetic from gas flow rate to final packed height without taking anything on trust.
If you have never sized a scrubber before, read the primer first, then move the inputs and watch what changes. The single most important thing to notice is this: gas flow sets the diameter, removal efficiency sets the height, and the two calculations barely speak to each other.
Orissa Engineering is an independent process engineering consultancy — process design, techno-economic evaluation and capital project execution for chemical plants.
How a scrubber works
What a scrubber is actually doing
A wet scrubber transfers a pollutant out of a gas stream and into a liquid. That is the whole job. The pollutant molecule has to travel from the bulk gas, through a thin stagnant gas film at the liquid surface, across the interface, through a thin liquid film, and into the bulk liquid — and it will only keep going if the liquid keeps its surface concentration low. Plain water does that by dilution, which runs out of steam quickly. A reagent does it by chemical reaction: caustic soda converts dissolved HCl into sodium chloride the instant it arrives, so the liquid surface is permanently hungry for more. That single difference is why a caustic scrubber is short and a water scrubber on the same duty is tall.
Two quantities decide the size of the vessel. How much gas you have to pass sets the cross-sectional area, because gas velocity through the packing cannot exceed the point at which upflowing gas stops the liquid from draining. How clean the outlet has to be sets the depth of packing, because each metre of packing removes a fixed fraction of what enters it, not a fixed amount. Confusing these two is the most common beginner error, and it is why a request like “make it 99.9% instead of 99% — can we just use a bigger column?” is the wrong question.
10.1 The four types you will meet
| Type | Gas removal | ΔP | Particulate tolerance | Plugging risk | Relative cost | Typical duty |
|---|---|---|---|---|---|---|
| Spray tower | Low–moderate, 1–2 NTU | 10–40 mmWC | Good (coarse) | Low | Low | Quench, coarse dust, first-cut acid gas |
| Venturi | Low for gas, 1 NTU at best | 150–1500 mmWC | Excellent, sub-micron | Very low | Moderate (power) | Fume, mist, sticky solids ahead of a packed bed |
| Packed bed | High, 3–10+ NTU | 15–50 mmWC/m | Poor | High | Low–moderate | Soluble / reactive gases to stack limits |
| Tray tower | High, discrete stages | 40–80 mmWC/tray | Moderate | Moderate | High | Slurry service, wide turndown, staged pH |
10.2 Choosing between them
10.3 The anatomy of a packed column
Almost every packed scrubber that works, and every one that fails, can be explained by the eighteen items below. Note how much of the drawing is liquid handling: the column shell is the cheap part, and the sump, pump, distributor, bleed and pH loop are where the design is won or lost.
10.4 Two-film theory, in one picture
The pollutant is dragged across two films in series, and like resistors in series the larger one decides the current. For a very soluble or chemically consumed gas, m is effectively zero, the second term vanishes, and the whole design collapses to “how well is the gas film stirred?” — which means gas distribution and interfacial area. For a sparingly soluble gas, m is large, the liquid term dominates, and no amount of gas-side turbulence will save you: you need reagent, more liquid, or more stages.
10.5 Which film controls, and why it matters
HCl in caustic. The reaction is instantaneous and irreversible. Chloride ion cannot come back out. Equilibrium back-pressure is nil, liquid-film resistance is essentially eliminated, and the column is gas-film controlled. Consequence: a very short bed does the job, and the thing that will actually hurt you is maldistributed gas short-circuiting up the wall — not bed depth.
SO₂ in caustic. Mixed control. The reaction is fast but the sulphite/bisulphite equilibrium and pH both matter. Drop below about pH 9 and the liquid film starts to reappear; the same hardware quietly loses efficiency with no mechanical change at all.
Cl₂ in weak or depleted liquor. Henry constant around 590 — chlorine does not want to be in water. With strong caustic (or hypochlorite) the reaction hides that, but the moment free alkali is exhausted the liquid film dominates and outlet chlorine climbs immediately. This is why the same geometry gives 99.9% on HCl and struggles on Cl₂, and why “we have a scrubber” is not the same statement as “we can scrub that”.
10.6 NTU — the difficulty number
NTU (number of transfer units) is a property of the separation you are asking for, not of the equipment. It is the answer to “how many times over do I have to renew the driving force to get from inlet to outlet?”. For a reactive system it is simply ln(1/(1−η)), which is brutally logarithmic:
| Required removal | 90% | 99% | 99.5% | 99.9% | 99.99% |
|---|---|---|---|---|---|
| NTU required | 2.3 | 4.6 | 5.3 | 6.9 | 9.2 |
| Relative bed depth | 1.0× | 2.0× | 2.3× | 3.0× | 4.0× |
Every additional nine costs you the same increment of packing as the first 90% did. That is the most useful single fact on this page: when a specification is tightened from 99% to 99.99% at no extra cost, someone is going to be disappointed.
10.7 HTU — the equipment quality number
HTU (height of a transfer unit, HOG here) is a property of the hardware and the operating point, not of the target. It is the depth of packing needed to accomplish one transfer unit. Better wetted area, higher gas-film coefficient, finer packing, a liquid rate that actually covers the packing — all reduce HTU. Typical HOG for random packing in gas absorption is 0.3–1.0 m; for fast reactive systems in fine packing it can fall below 0.3 m, which is why caustic scrubbers look suspiciously short to anyone used to distillation.
Z = HTU × NTU
NTU is what the regulator and the process ask of you. HTU is what your hardware can deliver. Multiply them and you have the packed height. Diameter never enters this equation — it came from the hydraulics, separately. If you remember nothing else from this page, remember that the two halves of a scrubber design are independent, and that a mistake in one cannot be fixed by the other.
10.9 Why more caustic does not always mean more removal
Raising NaOH strength feels like it must help — more reagent, more removal. It does not work that way. Concentrated caustic is viscous and has a higher surface tension, and both terms sit inside Onda’s wetted-area correlation. Higher σ against a plastic packing with a critical surface tension of only 0.033 N/m means the liquid beads instead of spreading, so the effective interfacial area aw falls; higher µL depresses the liquid-film coefficient kL. Meanwhile you generate more dissolved salt, more scaling and corrosion duty, and a larger effluent bill. The live sweep further down the page plots aw/at and HOG against caustic strength for your current design so you can see where the optimum sits.
10.10 Materials of construction
| Service | Shell / internals | Watch out for |
|---|---|---|
| Wet Cl₂ / HCl | FRP (vinyl ester), PP, PVDF, rubber-lined carbon steel | Never bare stainless — chloride pitting and SCC |
| SO₂ / SO₃ traces | FRP, PP, PVC below 60 °C | Acid dew point in the ducting upstream |
| Caustic liquor, ambient | Carbon steel with epoxy, FRP, PP | Caustic stress corrosion above 50 °C in CS |
| Hot or abrasive gas | Ceramic packing, refractory-lined inlet | Thermal shock on quench |
| Above 90 °C | Stainless with care, PVDF, FRP with high-temp resin | Plastic packing creep and bed collapse |
Note the connection back to mass transfer: polypropylene packing is chosen for chemical resistance and price, but its critical surface tension (0.033 N/m) is the lowest in the table in section 6.2, so it wets poorly. A ceramic or steel bed of identical geometry gives a noticeably higher aw/at and therefore a shorter column. Change the material selector in the calculator and watch HOG move — that path is live.
Preliminary sizing calculator
Everything recalculates as you type. Nothing is hidden: each result traces to an equation shown in the design walkthrough, and every constant is listed and editable in the assumptions panel.
| Species | In kg/h | Out kg/h | yin | yout | η % | NTU |
|---|
| Line item | ₹/h | ₹/day | ₹/year |
|---|
Design datasheet — every number the calculator produced
Six blocks, in the order the calculation runs. Blocks A and B fix the diameter from hydraulics; C and D fix the height from mass transfer; E and F follow from both. Nothing here is a lookup — every value comes from an equation shown in the walkthrough below, and all of it re-computes as you type.
The flooding and loading chart
Chart A — Pressure drop vs gas velocity
Three liquid loads: 0.5×, 1.0× and 2.0× your design irrigation rate. Notice that more liquid moves the flood point to the left — liquid and gas are competing for the same void. Shaded bands: pre-loading, loading (from 0.70 uflood) and flooding. The crosshair is your design point.
Chart B — Eckert generalised pressure-drop correlation
The flooding line is digitised from the published GPDC chart (table editable in the assumptions panel). Your design point sits below it; the vertical gap is your capacity margin, printed as % of flood.
What is actually happening in the bed
Liquid enters at the top and trickles down over the packing as a film, wetting whatever surface it can reach. Gas enters at the bottom and threads its way up through whatever void is left over. At low gas velocities the two phases barely notice each other. Pressure drop rises roughly with the square of velocity, exactly as it would in a dry packed bed, and the amount of liquid sitting in the bed at any instant — the holdup — depends only on the liquid rate. This is the pre-loading region, and it is where a well-designed absorber lives.
Push the gas rate up and the drag of the rising gas on the descending film stops being negligible. The film is slowed down, so more liquid has to be resident in the bed to pass the same flow — holdup increases. That liquid occupies void that the gas was using, so the gas has to squeeze through a smaller flow area, and pressure drop starts climbing faster than u². The kink where this begins is the loading point, conventionally taken here at 70% of the flooding velocity. There is a genuine irony in this region: mass transfer is often at its best just above loading, because the extra holdup means more interfacial area. Operating there deliberately is a specialist decision, not a default.
Keep pushing and the drag force on the liquid balances gravity. Liquid can no longer drain. It accumulates, the void fraction collapses, and the bed inverts from a gas-continuous system with liquid films to a liquid-continuous system with gas bubbles. Pressure drop goes near-vertical, liquor is carried out of the top of the column, and separation collapses. The critical operational point is this: a flooded scrubber does not degrade gracefully. It stops working, and it usually announces itself as liquid at the stack.
So why design at 60–70% of flood, when the chart says you could run at 90? Four reasons, all of them things that have happened on real plants. Turndown: the plant will run at 40% rate on a bad day and the distributor must still work. Upset flows: a relief case, a nitrogen purge left open, a batch that runs away, and suddenly the gas rate is 1.5× nameplate. Fouling: void fraction falls as solids and salts deposit, which raises the local velocity and lowers the flood point over the years. And the correlation itself: the GPDC flooding line carries roughly ±20% uncertainty, which means a paper design at 85% of flood can be a physical design at 105% of flood. Designing at 85% on paper is how columns come to flood the first week production exceeds nameplate.
Diameter is set by hydraulics. It comes from gas mass flux, gas and liquid densities, liquid viscosity and the packing factor. It does not know or care how much pollutant you are removing. Height is set by mass transfer. It comes from HTU and NTU, that is, from wetted area, diffusivities, the reagent chemistry and the outlet specification. It does not know or care about pressure drop. Change the required efficiency and only the height should move. Change the gas flow and both move, because gas flow enters the hydraulics directly and the mass transfer coefficients through the gas mass flux. The moment this distinction lands, scrubber design stops feeling like magic and starts feeling like arithmetic.
Health check on your own design: 15–50 mmWC per metre of packing is the band a well-proportioned atmospheric absorber sits in. Below about 8 mmWC/m the column is oversized and you have paid capital for nothing; the gas is barely touching the packing. Above about 80 mmWC/m you are into the loading region with no upset margin. The datasheet prints a verdict on this automatically.
The design walkthrough, with your numbers
Every step below regenerates from the current inputs. Statement of intent, the equation, the substitution, the result, and what it means. If a number in the datasheet ever looks wrong, this is where you find out why.
The caustic-strength optimum
aw/at and HOG vs NaOH strength
Swept at your current gas flow, column area and packing. Liquid density, viscosity and surface tension are interpolated from the property table in section 6.3 and extrapolated to 40 wt%. Reagent chemistry is assumed fast in all cases, so this isolates the hydrodynamic penalty of strong caustic.
Reading the curve
Wetted area falls monotonically as caustic strength rises, because both viscosity and surface tension work against spreading on a low-energy plastic surface. HOG therefore rises, and the column needs to be taller to do the same job. Against that, stronger liquor means a smaller circulating volume for the same reagent duty and a smaller effluent volume.
The practical conclusion from the source work: 10–20 wt% NaOH is usually the sweet spot for a recirculating scrubber. Below that you are pumping mostly water and the sump concentration swings with every batch; above it you are paying reagent to make mass transfer worse, then paying again to dispose of the salt. If you need the removal, buy it with bed height or a second stage — not with concentration.
Caveat worth stating plainly: this sweep captures the hydrodynamic effect only. It does not model reaction kinetics, so it cannot show the (real) benefit of high alkalinity when the reaction is not fast, nor the buffering advantage in a plant with violently variable load. It is a design insight, not a law.
Worked validation example — sulphur monochloride plant
The methodology on this page was developed against a real design case: the tail-gas scrubber for a 5 TPD sulphur monochloride (S₂Cl₂) plant, in which molten sulphur is chlorinated and the vent carries SO₂, HCl and unreacted Cl₂. The packed column is the second stage, downstream of a wet venturi. Press the button in the calculator to load every input from this case, then compare against the reported values.
Case definition
| Duty | Second-stage packed column, downstream of wet venturi |
| Design case | Extreme / failure case, not normal operation |
| Gas to column | 603.33 Am³/h, 35 °C, 0.5 kg/cm²g (≈1.484 atm) |
| SO₂ inlet | 100.40 kg/h (y = 0.141) |
| HCl inlet | 229.03 kg/h (y = 0.565) |
| Cl₂ inlet | 232.00 kg/h (y = 0.294) |
| Normal case | 12.52 Am³/h; SO₂ 1.87, HCl 4.27, Cl₂ 2.54 kg/h |
| Stack targets | Cl₂ 0.50, HCl 1.00, SO₂ 0.50, PM 2.5 kg/h |
| Medium | NaOH 20 wt%, 50% excess |
| Packing | 1 in polypropylene Pall rings |
| Column ID | 700 mm (A = 0.3848 m²) |
Calculated vs reported
| Parameter | Report | This page | Variance |
|---|---|---|---|
| Press “Load validation case” in the calculator to populate this comparison. | |||
The source report used a liquid viscosity of 2×10⁻⁶ Pa·s for 20 wt% NaOH and the same figure for the gas. Both are one order or more out: 20 wt% caustic at 35 °C is about 2×10⁻³ Pa·s, and a typical process gas is about 1.8×10⁻⁵ Pa·s. This page uses the corrected values, which changes ReL and WeL in Onda’s wetted-area term and therefore changes aw, kL and HOG. So the HTU printed here will not exactly reproduce the reported 0.241 m, and it should not. Constants have not been tuned to force a match. A stated 10–30% variance with a physical explanation is a more useful engineering document than a suspiciously exact reproduction — and reconciling that variance is exactly the work a design review consists of.
The other reconciliation worth understanding: the report’s 3.83 m³/h is the reagent solution consumption derived from stoichiometry. It is not the pump duty. A real scrubber recirculates roughly 10–30 times that from the sump with a continuous bleed. The calculator prints both numbers separately, because one drives operating cost and the other drives the pump and the distributor.
Assumptions, correlations and limits
Correlations used, with sources and validity ranges
| Purpose | Correlation | Source | Validity / caveat |
|---|---|---|---|
| Flooding capacity | Generalised pressure-drop correlation, K₄ form | Eckert; Sherwood–Leva; presented in Coulson & Richardson Vol. 6 (Sinnott) | Random packing, countercurrent, non-foaming. Flooding line read from a digitised chart — ±20% at best. |
| Flood ΔP cross-check | ΔPflood = 0.115 Fp0.7 | Kister & Gill | Fp in ft⁻¹, result inch H₂O per ft. Valid Fp < ~60 ft⁻¹. |
| Dry bed ΔP | Ergun equation | Ergun (1952) | Written for beds of solid particles. For hollow rings and saddles it needs an equivalent diameter, and the answer is sensitive to which one you choose: the nominal size (used here, as tabulated in the packing database) under-predicts, while the surface-equivalent 6(1−ε)/at over-predicts. Both are printed. Vendor pressure-drop curves normally fall between them — use them for fan sizing. |
| Operating holdup | hL = (12 µL uL at² / ρL g)1/3 | Billet & Schultes, laminar film form | Pre-loading region only. Ignores the gas-rate dependence of holdup above loading. |
| Irrigated ΔP | Void-reduction factor (ε/(ε−hL))³ | Standard practice | Pre-loading only. |
| Loading-to-flood escalation | [1 − (u/uflood)⁴]−0.5 | Interpolation for illustration — not a rigorous correlation | Used only to give Chart A a physically sensible shape between loading and flooding. Do not quote it. |
| NTU, reactive | NOG = ln(1/(1−η)) | Limiting case of the Colburn equation as m → 0 | Requires fast, irreversible reaction and adequate free reagent at the interface. |
| NTU, physical | Colburn equation | Colburn (1939) | Dilute systems, straight equilibrium line, isothermal. |
| Wetted area, kG, kL | Onda et al. correlations | Onda, Takeuchi & Okumoto (1968) | Random packing 4–50 mm, ReL 0.04–500. Predicts aw to about ±20%; sensitive to σ and σc. |
| Gas diffusivity correction | DG ∝ T1.75/P | Fuller et al. | Referenced to 25 °C, 1 atm. |
| Liquid diffusivity correction | DL ∝ T/µL | Stokes–Einstein | Referenced to 25 °C in water. |
| Droplet size (venturi) | Nukiyama–Tanasawa | Nukiyama & Tanasawa (1938) | Air–water fitted; CGS units internally. |
| Venturi efficiency | Johnstone, η = 1 − exp(−k R √Ψ) | Johnstone et al. | k is empirical, 0.1–0.2. Single droplet size, no size distribution. |
| Venturi ΔP | ΔP = 1.03×10⁻³ uth² R | Calvert | uth in cm/s, R in L/m³, result cm H₂O. |
Digitised GPDC flooding line — editable, recalculates live
Digitised from the generalised pressure-drop correlation (Eckert / Sherwood–Leva, as presented in Coulson & Richardson Vol. 6). Verify against your own copy of the chart before using for detailed design. Interpolation between points is linear in log–log space. Edit the JSON and press Apply; everything on the page recomputes.
Editable constants block
Every non-input constant the engine uses. Edit and apply — results, charts and walkthrough all follow.
Random packing database
Rows marked extended are typical vendor values added so the tool is useful beyond the source case; they are not from the source report. Always replace with the vendor’s own Fp and at for a real enquiry — packing factors vary by 20% between makers for nominally identical shapes. Unit reconciliation: the tabulated packing factors (57.5 for a 1 in Pall ring, 150 for a 1 in Raschig ring) are labelled m−1 in the source but match published ft−1 values almost exactly. Used as m−1 in Sinnott’s K₄ equation they over-predict the flooding velocity by roughly 80% and produce absurdly small columns. This page therefore reads the tabulated column as ft−1 and multiplies by 3.281 to obtain the value used in the correlation; both are shown above, and the Fp field in panel 5 is editable so you can reproduce either interpretation.
Scrubbing medium properties and packing material σc
Deliberate correction to the source data. The source report used µL = 2×10⁻⁶ Pa·s for 20 wt% NaOH and µG = 2×10⁻⁶ Pa·s for the gas. Both look like unit slips of one order or more: 20 wt% NaOH at 35 °C is ≈2×10⁻³ Pa·s and a typical process gas is ≈1.8×10⁻⁵ Pa·s. The corrected values are used as defaults here because liquid viscosity enters Onda’s wetted-area and kL terms and gas viscosity enters both the Ergun equation and kG — a factor of 1000 in µL is not a rounding difference.
Reagent stoichiometry and transport properties
CO₂ is included as a parasitic consumer. Any caustic scrubber handling an air-diluted stream absorbs CO₂ continuously, which consumes reagent and drives up dissolved carbonate whether you want it or not. The tool warns about it; it does not model the rate.
What this tool does not do
No rigorous vapour–liquid equilibrium. No rate-based or stage-based simulation. No temperature profile along the column and no heat of absorption — strongly exothermic duties (concentrated HCl, SO₃) will run hotter than assumed and lose capacity. No multicomponent interaction between solutes. No CO₂ absorption from air. No foaming or fouling factors. No structured packing. No mechanical design: shell thickness, nozzle sizing, distributor orifice layout, support-plate selection, demister selection and hold-down design are all out of scope. No materials selection. No reaction kinetics, so systems with genuinely slow reactions (NO, chelated NOx, organic odours) will be optimistic.
Validity envelope: atmospheric to 3 bar(a); 5–80 °C; dilute systems below roughly 10 mol% solute; random packing only; countercurrent only; single liquid feed point per bed; non-foaming liquor.
Ten ways scrubber designs go wrong
Efficiency has nothing to do with diameter. Diameter comes from gas mass flux against the flooding limit. If someone widened the column to get more removal, they have made the gas velocity lower, the wetting worse and the removal worse.
Vent scrubbers spend their lives on transients: batch charging, reactor runaway, relief, purge. Size on the credible worst case and check the steady case for turndown, not the other way round. The validation case on this page is deliberately an extreme case for exactly this reason.
Any air-diluted stream carries 400+ ppm CO₂, and caustic absorbs it happily and irreversibly into carbonate. On a large dilute stream this parasitic duty can exceed the duty you designed for. Either accept the cost, switch to a milder reagent such as sodium carbonate or bicarbonate, or control pH lower.
Below MWR the film breaks into rivulets. Dry patches carry gas straight through, and the bed keeps looking perfectly healthy from outside — same ΔP, same level, same pump amps. This tool refuses to let the design go below MWR and warns loudly.
Stoichiometry tells you how much reagent is consumed. Hydraulics tells you how much liquid must be pumped over the bed to wet it. These differ by an order of magnitude. Both are printed separately in the datasheet above.
Liquid migrates to the wall in about 10 column diameters of packing. After that the centre of the bed is dry and the gas knows it. Split the bed and redistribute; the tool flags the split when Z/D exceeds 10.
Entrained droplets carry dissolved salt at sump concentration. A stack test then reports an emission you never generated in the gas phase. Check the demister velocity against its own flooding limit, and provide a wash header.
It is the cheapest instrument on the package and the only one that tells you the bed is fouling. Trend it. A slow rise in ΔP at constant gas rate is your maintenance warning; a sudden fall usually means the bed has channelled or the support plate has failed.
The correlation has ±20% uncertainty and the plant has more than that in flow variability. 85% on paper is 100%+ in the field. Keep it at 60–70% and spend the extra 100 mm of diameter.
Packing is a filter if you let it be one. Solids, sublimed product, polymer, sulphur mist — all of it lodges in the bed, and no amount of wash-down recovers a channelled bed. Venturi or spray quench first, always.
Glossary and nomenclature
| Symbol | Unit | Meaning |
|---|---|---|
| A | m² | Column cross-sectional area (also absorption factor Lm/mGm in the Colburn equation) |
| at | m²/m³ | Total specific surface area of the packing |
| aw | m²/m³ | Wetted (effective interfacial) area — the area that actually does mass transfer |
| CT | kmol/m³ | Total molar concentration of the liquid |
| D | m | Column internal diameter |
| dp | m | Equivalent packing diameter (nominal size) |
| dd | µm | Sauter mean droplet diameter in the venturi throat |
| DG, DL | m²/s | Diffusivity of the solute in gas and in liquid |
| FLV | – | Flow parameter, (Lw/Vw)√(ρG/ρL) |
| Fp | m⁻¹ | Packing factor — the empirical resistance of the packing to flow |
| G, L | kg/m²·s | Gas and liquid mass flux |
| Gm, Lm | kmol/m²·s | Gas and liquid molar flux |
| hL | m³/m³ | Operating liquid holdup |
| HG, HL | m | Height of a gas-film / liquid-film transfer unit |
| HOG | m | Height of an overall gas-phase transfer unit (HTU) |
| HETP | m | Height equivalent to a theoretical plate |
| K₄ | – | Eckert capacity function in Sinnott’s form |
| kG | kmol/m²·s·kPa | Gas-film mass transfer coefficient |
| kL | m/s | Liquid-film mass transfer coefficient |
| m | – | Equilibrium (Henry) slope y*/x. Zero in reactive mode |
| MWR | m³/m²·h | Minimum wetting rate of the packing |
| NOG, NTU | – | Number of overall gas-phase transfer units |
| QG, QL | m³/h | Gas and liquid volumetric flow |
| ReL, FrL, WeL | – | Liquid Reynolds, Froude and Weber numbers in the Onda correlation |
| us | m/s | Superficial gas velocity (volumetric flow / empty column area) |
| uth | m/s | Venturi throat velocity |
| Vw* | kg/m²·s | Gas mass flux at flooding (Eckert) |
| y, x | – | Solute mole fraction in gas and liquid |
| Z | m | Packed height |
| ε | – | Packing void fraction |
| η | – | Fractional removal efficiency |
| µG, µL | Pa·s | Gas and liquid dynamic viscosity |
| ρG, ρL | kg/m³ | Gas and liquid density |
| σ | N/m | Liquid surface tension |
| σc | N/m | Critical surface tension of the packing material |
| Ψ | – | Inertial impaction parameter (venturi) |
| νi | mol/mol | Stoichiometric reagent requirement per mole of pollutant |
Words you will hear on site
Loading point. The gas velocity at which liquid holdup starts to rise with gas rate. Pressure drop steepens here.
Flooding. The gas velocity at which liquid can no longer drain. The bed fills and the column stops separating.
Turndown. The ratio of maximum to minimum flow over which the column still works. Usually limited at the bottom by the distributor and the minimum wetting rate, not by the packing.
Channelling. Gas or liquid finding a preferred path, usually the wall. The enemy of every packed bed.
Bleed / purge. Continuous removal of liquor to stop dissolved salts accumulating. Its rate sets your effluent bill.
ORP. Oxidation–reduction potential. On hypochlorite scrubbers it is the measurement that tells you whether oxidant is present, which pH alone cannot.
MWR. Minimum wetting rate. Below it the packing film breaks up and efficiency falls off a cliff.
Irrigation density. Liquid flow per unit column area, m³/m²·h. The number to quote when comparing two designs.