Absorption Column Calculator
Design a packed absorption column: calculate diameter, packing height, number of transfer units (NTU), and pressure drop.
About this calculator
A packed absorption column removes a soluble gas component (like CO2, H2S, or an organic vapor) from a gas stream by contacting it with a liquid solvent flowing the opposite direction through a bed of packing material that maximizes gas-liquid contact area. The absorption factor A = L/(mG) -- liquid flow divided by the product of Henry's Law constant and gas flow -- captures the fundamental tradeoff in every column design: a smaller Henry's constant (more soluble gas) or a higher liquid-to-gas ratio both make absorption easier, which the Kremser/Colburn equation this calculator uses translates directly into fewer required theoretical transfer units (NTU) for the same separation duty. Packing height is simply NTU multiplied by HTU (height of a transfer unit, the physical column height that delivers one theoretical unit of mass transfer) -- so a column needing more NTU, or packing with a taller HTU, both demand a taller vessel.
There's also a hard physical limit this calculator doesn't fully model: when the absorption factor A drops below 1, no column height -- however tall -- can drive the outlet gas concentration below the "pinch" value A times the inlet concentration, because the liquid simply doesn't have enough capacity relative to the gas flow to absorb further; real column design always keeps A safely above 1 (values of 1.4-2.0 are common rules of thumb) specifically to stay well clear of that pinch. Column diameter and pressure drop here are simplified, illustrative estimates rather than a published sizing correlation -- real packed-column diameter sizing uses flooding correlations (like the generalized pressure-drop correlation) that account for both gas and liquid properties together, and a vendor's packing-specific data should always be the final word for an actual design.
Inputs
Results
Column Diameter
0.49 m
≈ 6 credit cards
Packing Height
1.7 m
≈ 11 smartphones
How to Use This Calculator
- Enter the Gas Flow Rate in m³/hr and the Inlet and Outlet Gas Concentrations as mole fractions to define the required absorption duty.
- Enter the Liquid Flow Rate in m³/hr — the liquid-to-gas ratio controls absorption efficiency and solvent consumption.
- Enter Henry's Law Constant (m) for the gas-solvent system at operating temperature — larger values mean less soluble gas and require more transfer units.
- Enter the Packing Factor in 1/m from the packing vendor's datasheet (Raschig rings, structured packing, etc.).
- Review Column Diameter in meters and Packing Height in meters to size the vessel and estimate capital cost.
- Check the Number of Transfer Units (NTU) and Height of Transfer Unit (HTU) to verify sufficient mass transfer capacity.
How the result changes with Gas Flow Rate
| Gas Flow Rate | Column Diameter | Packing Height |
|---|---|---|
| 500 | 0.34 m | 1.12 m |
| 750 | 0.42 m | 1.38 m |
| 1,500 | 0.6 m | 2.58 m |
| 2,500 | 0.77 m | 8.33 m |
What each input means
- Gas Flow Rate
- Volumetric flow rate of the gas entering the column.
- Inlet Gas Concentration
- Mole fraction of the solute in the inlet gas stream.
- Outlet Gas Concentration
- Desired mole fraction of solute in the exit gas (must be less than inlet).
- Liquid Flow Rate
- Volumetric flow rate of the absorbing liquid.
- Henry's Law Constant (m)
- Henry's Law constant relating gas-phase to liquid-phase concentration (y = m·x).
- Packing Factor
- Characteristic factor for the column packing type (e.g., Raschig rings ~580, Pall rings ~170).
How this is calculated
Worked example, using the default values
- Identify Input Parameters6 parametersGas Flow Rate = 1000, Inlet Gas Concentration = 0.05, Outlet Gas Concentration = 0.005, Liquid Flow Rate = 2000, Henry's Law Constant = 0.8, Packing Factor = 100 = 6 input(s) provided
- Calculate Column DiameterColumn Diameter = Math0.486 = 0.486
- Calculate Packing HeightPacking Height = Math1.702 = 1.702
- Calculate Number of Transfer UnitsNumber of Transfer Units = Math3.094 = 3.094
- Calculate Height of Transfer UnitHeight of Transfer Unit0.55 = 0.55
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does a higher liquid flow rate reduce the number of transfer units needed?
More liquid relative to gas raises the absorption factor A = L/(mG), which means the liquid has more spare capacity to pull the solute out of the gas stream at any point in the column. A column operating with a higher A approaches its separation target more efficiently per unit of packing height, so fewer theoretical transfer units are needed to hit the same target outlet concentration -- though more liquid also means more solvent to circulate and regenerate, so real designs balance the two.
What is the 'pinch' this calculator's number of transfer units can hit?
When the absorption factor A (liquid flow divided by Henry's constant times gas flow) drops below 1, the liquid entering the column can't remove enough solute to drive the exit gas concentration below A times the inlet concentration, no matter how many theoretical stages or how tall the packing -- that floor is the pinch. Real column design keeps A comfortably above 1 for exactly this reason; operating near or below A=1 makes the separation target physically unreachable rather than merely expensive to reach.
Why does a larger Henry's Law constant mean more transfer units are needed?
Henry's Law constant relates gas-phase to liquid-phase concentration at equilibrium (y = m x) -- a larger m means the gas holds onto the solute more strongly relative to how much the liquid can absorb at equilibrium, i.e. the gas is less soluble in that particular solvent. Since the absorption factor A = L/(mG) shrinks as m grows, a less-soluble gas needs a correspondingly higher liquid rate or more transfer units (taller packing) to achieve the same separation.
Are the column diameter and pressure drop figures precise enough to build from?
Treat them as first-pass sizing estimates, not final engineering numbers -- this calculator's diameter figure assumes a single representative gas velocity rather than the full flooding correlation (which depends on both gas and liquid density, viscosity, and the specific packing's characteristics), and pressure drop is a simplified proxy rather than a vendor pressure-drop chart. Real detailed design should use the packing manufacturer's generalized pressure-drop correlation data for the specific packing selected.
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