Absorption Column Height Calculator
Determine the required packed bed height for a gas absorption column using mass transfer coefficients and equilibrium data. Used by process engineers designing scrubbers and gas treatment systems.
Last updated: September 2026
Formula below · 2 sources (aiche.org, Wikipedia) · Updated Sep 2026
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About this calculator
Packed column height Z is found by dividing the number of transfer units (NTU) by the height of a transfer unit (HTU). NTU represents the difficulty of separation and is derived from the log ratio of inlet to outlet driving force. This calculator uses the dilute-limit form of the NTU-based packed-column height equation: Z = (G/3600) × ln(C_in / C_out) / (K_oa × A_c), where G is gas flow (m³/h), C_in and C_out are inlet and outlet gas concentrations, K_oa is the overall volumetric mass transfer coefficient K·a (1/s, i.e. a film coefficient in m/s times the interfacial area per packed volume in m²/m³), and A_c = π·D²/4 is the column cross-sectional area. This form is valid when the liquid stream enters as near-pure solvent (equilibrium partial pressure of solute at the liquid inlet ≈ 0), which covers most industrial absorbers designed with lean incoming solvent — Henry's law constant drops out of the driving force in this limit, so it is not one of this calculator's inputs. Taller columns or larger diameter improve separation efficiency but at higher capital cost. For non-dilute systems with a non-negligible inlet equilibrium concentration, the full log-mean driving-force form (which does depend on the equilibrium constant) is required instead.
How to use
Inputs: G = 360 m³/h, C_in = 0.05 mol/m³, C_out = 0.005 mol/m³, K_oa = 0.01 1/s, D = 1.5 m. Cross-section: A_c = π × 1.5² / 4 = 1.767 m². G in m³/s = 360 / 3600 = 0.1 m³/s. ln(C_in / C_out) = ln(0.05 / 0.005) = ln(10) = 2.303. Z = 0.1 × 2.303 / (0.01 × 1.767) = 0.2303 / 0.01767 ≈ 13.03 m.
Frequently asked questions
What is the overall mass transfer coefficient and how is it measured for an absorption column?
The overall volumetric coefficient K_oa (here in 1/s) is a film mass-transfer coefficient (m/s) multiplied by the wetted interfacial area per unit packed volume (m²/m³). It lumps together resistance in the gas and liquid films and the packing's active area. Dividing the superficial gas velocity (G/A_c, m/s) by K_oa gives the height of a transfer unit in metres. It is typically determined experimentally on a pilot column, then back-calculated from the NTU-HTU design equation; it depends on packing type and size, gas and liquid loads, physical properties and temperature. Packing vendors publish K·a data as a function of gas and liquid loading. Molar forms (kmol/(m³·h·mole fraction)) convert by dividing by the gas molar density.
Why doesn't this calculator ask for Henry's law constant?
This calculator uses the dilute-limit driving force, ln(C_in / C_out), which assumes the liquid enters as near-pure solvent — the standard case for a lean absorbent. In that limit, the equilibrium gas-phase concentration in contact with the incoming liquid is essentially zero, so Henry's law constant (which relates equilibrium gas- and liquid-phase concentrations, y* = m·x) cancels out of the driving-force ratio and has no effect on the required height. It only re-enters the calculation for non-dilute systems, where the liquid already carries a meaningful concentration of solute at the inlet — a case this simplified tool does not cover.
Why is packed column diameter selected separately from column height in absorption design?
Column height determines the number of transfer units needed to achieve the desired separation — it is set by mass transfer kinetics and equilibrium. Column diameter, on the other hand, is set by hydraulic constraints: the cross-sectional area must be large enough to accommodate the gas and liquid flow rates without flooding or excessive pressure drop through the packing. Flooding occurs when gas velocity is too high and prevents liquid from flowing down the packing. Engineers use generalized pressure drop correlation (GPDC) charts to find the maximum allowable gas velocity, then set diameter to operate at 60–80% of flooding velocity.